Sennheiser HD 660 S


100+ Head-Fier
Sennheiser HD660s by WaveTheory
Pros: a well-balanced and enjoyable sound signature; soundstage height; maintains Senn 6?? series comfort
Cons: The technical improvement over HD6XX or DT880 is not proportional to the $300 price difference; Using a tube amp doesn’t do much

It’s time to come back down to reality after a brief (but fun!) foray into high-end headphone systems. The Sennheiser HD660S headphone crossed my desk recently and I had a good listen. Let’s see what I found…


The Senn HD660S is a very good ~$200-250 headphone that unfortunately sells for $500 from most retailers. It has its strengths and offers a slightly different signature than its popular sibling model the HD6XX from Massdrop. It’s more forgiving than its 6XX sibling of entry-level DACs and amps. But, in this reviewer’s humble opinion it does not offer a signature that is different enough or a technical performance improvement over the 6XX, or another budget heavyweight Beyerdynamic DT880 600Ω, large enough to justify its ~$300 price difference over those models. However, if you can find it used around $250 or less, it can be a compelling option.


My preferred genres are rock/metal and classical/orchestral music. I’m getting to know jazz more and enjoying quite a bit. I also listen to some EDM and hip-hop. My hearing quirks include a high sensitivity to midrange frequencies from just under 1KHz to around 3Khz, give or take. My ears are thus quick to perceive “shoutiness” in headphones in particular. I describe “shoutiness” as an emphasis on the ‘ou’ sound of ‘shout.’ It’s a forwardness in the neighborhood of 1KHz and/or on the first one or two harmonics above it (when I make the sound ‘ooooowwwww’ into a spectrum analyzer the dominant frequency on the vowel sound is around 930Hz, which also means harmonic spikes occur again at around 1860Hz and 2790Hz). In the extreme, it can have the tonal effect of sounding like a vocalist is speaking or singing through a toilet paper tube or cupping their hands over their mouth. It can also give instruments like piano, but especially brass instruments, an added ‘honk’ to their sound. I also get distracted by sibilance, or sharp ‘s’ and ‘t’ sounds that can make ssssingers sssssound like they’re forssssssing esssss ssssssounds aggresssssssively. Sibilance does not physically hurt my ears nearly as quickly as shout, though. It’s distracting because it’s annoying and unnatural. Finally, I’m discovering that I have a preference for more subtle detail. I like good detail retrieval and hearing what a recording has to offer, but I prefer what many would consider relaxed and subtle rather than aggressive or detail-forward. To my ear, more subtle detail-retrieval sounds more realistic and natural than aggressive, detail-forwardness. There is a balance here, though, because detail retrieval can get too relaxed and that can sound unnatural, as well, or simply leave out important aspects of the recording. Readers should keep these hearing quirks and preferences in mind as they read my descriptions of sound.


The HD660S is an open-back, dynamic-driver, over-the-ear headphone. It has a rated impedance of 150Ω and rated sensitivity of 104 dB/mW. On paper, it’s an easier-to-drive load than most of the Sennheiser HD6?? series, which have impedances of 300Ω. As far as build quality goes, there’s not much to say here that hasn’t already been said. If you’re familiar with the Senn 600 series build, that’s what you get. I’m not going to spend too much time here because descriptions of the build are all over the internet. The time here will be better spent on the…


Test Gear

There were 3.5ish different signal chains I listened to the 660s with most. To acclimate, I used the Chord Hugo 2 DAC/amp (as DAC only) and the Cayin HA-1AMK2 tube amp on its lowest output impedance setting. That’s a source chain that’s probably a bit unrealistic for most users who may be considering the 660s, so I also did a fair amount of critical listening with the Schiit Modius + Asgard 3 stack, and a Topping D10 + JDS Labs Atom and Monolith Liquid Spark amp. Read about some of my thoughts on Asgard 3, Atom, and Liquid Spark amps here.

Sound Signature

To my ear the signature of the 660s is a gentle ‘v’…kinda. Why kinda? There’s not much subbass. The middbass is a bit forward, the mids are slightly recessed, and the lower treble is a bit forward. It’s kind like a radical symbol: √ - sorta, but the treble isn’t noticeably forward of the bass. If you’re familiar with the HD6XX, pull back the mids a touch and bring the lower treble up just a hair. The result is not as mid-focused or overall neutral-warm like the 6XX is, it’s a bit closer to a more mainstream tuning, but not nearly as extreme. It’s bass and treble extension are both somewhat mediocre, not going very deep or having much air, but that’s par for the course for Senn 6?? series cans. And like the other Senn 6?? cans, the 660s is generally smooth, relaxed, and easy to listen to for long periods of time without getting fatigued.

Detail Retrieval

Have you heard the phrase “Sennheiser veil”? Senns are not detail-forward so to many they can sound veiled when they are first put on. It took me awhile to drop this feeling after doing weeks of listening to the likes of the Audeze LCD-24, HiFiMan HE1000V2, and Abyss Diana Phi. However, Senns are actually quite detailed at their price points; they’re just subtle about it. Once that veiled feeling wears off, there’s a fair amount going on. The 660s is no exception here. It’s reasonably resolving in the mids and lower treble in particular. Room reverb, many vocal subtleties, etc. are presented well.

Spatial Presentation

The spatial presentation is still very Senn HD6??-like. It’s not particularly wide, its imaging tends toward the “3-blob” presentation (stuff left, stuff center, stuff right, not as much in between as some others), and there’s not a ton of depth. None of those things are deal breakers for most, so they won’t be here either. The 660s does a nice job of creating some more vertical space than some other headphones I’ve heard in the price range, though.

But If You Tube It

Yeah, it’s a Senn 6?? series, so we gotta talk about tube amps. At 150Ω, the 660S is basically the minimum impedance that makes sense to use on an OTL (output transformer-less) tube amp. Unfortunately, I don’t have a true OTL tube amp, I have the transformer-coupled Cayin HA-1AMK2. The 660s did indeed sound better on the 1AMK2 than it did on the Atom, Liquid Spark, or Asgard 3, but I don’t think that had much to do with it being a tube amp and the 660s taking well to tubes. If anything, it’s because the 1AMK2 is on a much higher performance tier than any of those amps. The truth is, the 660s did not change its sound much. Yes, there was slightly more detail and an improvement in overall technical performance, but there wasn’t the transformation that happens when the 6XX is put on a tube. So, you can use the 660s with a tube. It does change a bit. But, it’s not a completely different headphone on a tube in the way that its 600 and 650/6XX brethren are.

The 660s is a fine-sounding headphone. I can listen to it and enjoy it for extended periods of time. Nonetheless, this sound section is a bit shorter than I often do because I think the big story with the 660s is found in the…


I keep a Massdrop + Sennheiser HD6XX (which is essentially a Massdrop-branded HD650) and a Beyerdynamic DT-880 600Ω around as references pieces. Both of these cans are near $200 USD in MSRP and both are well-known, popular cans. They make excellent references for review work. I’ll go ahead and drop the punchline of this section here: the 660s has its own sound signature that may attract some listeners but it also is not a marked technical improvement over the 6XX or the DT880 – if it’s even an improvement at all. It also doesn’t scale up nearly as well. Both the 6XX and DT880 have something new to give when they’re driven by $1000+ amps and DACs. The 660s hits its performance ceiling much earlier. On the flip side, I found the 660s to be slightly – and I emphasize slightly – more forgiving with entry-level gear than either 6XX or DT880. I interpret this to mean it is a little less source-picky than these other two. Alright, let’s unpack all of this…

Sound Signatures

As stated before, the 660s has a very mild and somewhat rolled-off-at-either-end v-signature. The 6XX has a neutral-warm signature that is also somewhat rolled-off with mediocre extension both high and low. The DT880 is neutral-bright with excellent bass and treble extension but also somewhat lean bass. The amount of treble in the 660s is between the 6XX and DT880, but closer to the 6XX than the 880. The 660s and DT880 are quite similar in midrange quantity. The 6XX and 660s have similar amounts of midbass, being slightly more than the DT880, but the DT880 has the extension advantage and more subbass presence overall, despite it being somewhat lean in the lows (at least compared to its treble). I think the DT880 also has more bass punch/slam. It’s more dynamic. I wouldn’t say it has great punch/slam because of the lean-ness of its bass, but it is overall more energetic and aggressive throughout the frequency range than either Senn.

Spatial Presentations

The DT880 is the spatial king at its price point and its abilities in this area keep it competitive way beyond its price point. It’s well ahead of either Senn model in soundstage size, imaging, separation, and depth. After that, and in a somewhat shocking discovery for me given the intimate nature of the 6XX’s spatial presentation, the 660 is more horizontally narrow than the 6XX. It’s not a huge difference, but it’s noticeable. The 660s is also actually more 3-blob-y than the 6XX – another shocker. However, as mentioned before, the 660s handles vertical space much better than 6XX, creating a much more convincing sense of soundstage height.

Detail Retrieval

The DT880 initially comes across as the most detailed of the three because of its brightness and more aggressive presentation. Upon closer listening, it maintains a slight edge over either Senn in detail retrieval in the bass and the treble. The 6XX and the 660s are slightly more detailed in the midrange than the DT880. The 6XX probably is the one with more resolution in the mids, and the 660s is the one that’s smoother and more forgiving. Both of these qualities have advantages in certain situations.

When To Use These Headphones

The 6XX and DT880 both are a bit on the picky side. The 6XX is very transparent in the midrange and will wander into shouty/honky territory if its source chain (amp and DAC) aren’t strong in the mids. I learned this the hard way the first time I owned a 6XX. I was using a DAC that didn’t do so well in the mids from its single-ended output and that translated into shouty and honky mids, and was not particularly enjoyable. The DT880 has similar challenges in the treble. On a source chain that doesn’t handle treble very well, it will get sharp and shrill to the point of piercing sometimes. I did not notice such issues with the 660s. It sounds pretty good – probably the best of these 3 headphones – on the ~$75ish (and ESS-based!) Topping D10 DAC, and Atom ($99) or Liquid Spark ($109) amps. The midrange is smoother and more natural on that stack with 660s than the 6XX and the treble is pleasant and reasonably detailed where the DT880 can get over-the-top sharp (especially on Atom).

Moving up to the Modius + Asgard 3 pile of Schiit, the 3 headphones are on a roughly equal technical level across the board and signature preference will make the determination as to which one is the favorite. On this stack, the DT880’s sharpness is largely tempered and the 6XX’s mid-shout is virtually absent. IMO, the Asgard 3 in particular is the entry-point for when the 6XX and DT880 should be reasonably considered. The DT880 can sound pretty good on the Liquid Spark most of the time, but still gets a bit shrill even on that warmer amp. With the Asgard 3 its treble sharpness will be a non-issue for all but the most treble sensitive listeners. The 6XX’s mids really need the warm, steady hand of the Asgard 3, though. The 660s is quite enjoyable and competent. It has its own signature that some will find appealing. But, at $500 it’s not yet separating itself in technical proficiency from either of the other $200 headphones.

It's when the source chain moves into the (multi)kilobuck range where it becomes difficult to justify the 660s costing $300 more than 6XX or DT880. On the Hugo 2 + HA-1AMK2 combo, the 6XX’s mids sound very detailed and organic, with stunningly natural timbre considering the price point. The DT880 musters hints of meaningful bass texture and has treble timbre every bit as good as what the 6XX delivers in the midrange. The 660s sounds better than it did on the $400 Schiit-pile but does not sound as technically proficient as the 6XX or DT880 at this level. It lacks a standout feature that the other two bring.

What does this mean? It’s tricky. My read on this is the 660s is a mid-fi-priced headphone that should be priced more in the $200-250 range. It would be a solid headphone at that price and wow many audiophile newbies. While I’ve not heard it, my understanding is the Massdrop + Sennheiser HD58X kinda is the 660s…almost…at $185. The prices of the 6XX and DT880 are also misleading. They are priced as entry points into the headphone-enthusiast game, but they require a healthy investment in the electronics that drive them to hear what makes them so special. They are mid-fi headphones priced at the entry-fi (for enthusiasts, anyway) level. Some of that pricing is because they’ve been around for a long time and so paying for development cost is no longer necessary. The Senn HD650 was $500ish once upon a time, and so was the DT880 long ago. [The core DT880 design has been around since 1980!] Nonetheless, the 660s is a fine-sounding headphone that seems to be struggling for a true place in the market. Yes, it’s signature is different, but not that different. It’s a fine headphone that doesn’t really have a niche, IMO.

And then there’s tubing. A great party trick the 6XX has is that it’s essentially 2 headphones. It has one signature for solid state amps – which is pretty good – and then really wakes up on a tube amp with more alive bass and treble. The DT880 isn’t quite as dramatic but also changes its behavior significantly between solid state and tube amplification. The 660s doesn’t change as much as either of these other two. Does it sound good on tube amps? Yes. It does. It sounds good on solid state amps too. It just doesn’t transform to the degrees that other Senn 6?? do and doesn’t match the difference the DT880 gives either.


The above probably makes it sound like I don’t like the HD660s. That’s not true. I genuinely enjoyed its sound when I listened to it on its own merits. I spent several dozen hours with it before comparing it against the HD6XX or the DT880. I enjoyed that time. I really did. I was pleasantly surprised. But then I did put on the HD6XX and DT880. And every time I go back to one of those two I think “why don’t I listen to these more?” The effect is not the same with the 660s because it doesn’t scale up like 6XX or 880. At that point I started looking at prices and numbers. It’s a total of approximately $675 for the HD660s, Topping D10 (now D10s), and JDS Labs Atom. It’s $620 for Schiit Modius, Schiit Asgard 3, and HD6XX or $600 if you swap the DT880 for the 6XX. The experience with the Schiit stack and 6XX/880 is better than 660s + D10 + Atom. The experience with the 660s + Schiit stack is about the same in overall quality as with Schiit stack + 6XX/880 but is also now $900 instead of $600ish. At that point it’s more cost-effective to get the Schiit stack + 6XX/880 and another complementary headphone for $300. And then either the 6XX or 880 will stay relevant in your collection several upgrades down the road when this hobby inevitably leads you to thousand-dollar or more amps and DACs. The 660s will run out of tricks up its sleeve.

For me the bottom line is the 660s is an enjoyable headphone that makes more sense if you can land it for ~$250. Even $300 is pushing it, IMO. It has a different signature than DT880 or its HD6XX sibiling, but not different enough in my view to justify $300 more for it. It’s an enjoyable headphone that appears to struggle to make its own unique mark on the market and justify its $500 price tag.

Thanks for reading another long review, all. Enjoy the music!
If u preferring m40x then i bealive none of these audiophile open backs are for u.
Other than that agree 660s sound better than many 1.000 euro headphones. 6XX is a good option if u can get them at low price but to me 660s are better in every way. U have to know that Headphones doesnt work like the more u give the more u get... To some point u pay 3 times the price for 5% more.
If i understand correct u want bass and energy closed back... Get the pioneer HDj series or V moda series search something from those line ups.
Ehy Tripokaridos, yep i like my energetic closed back, but thats why now i am thinking on getting something different! So maybe hd6xx because hd660 just seems ovepriced
For the price yeah 6xx. For the best 660s.

Dobrescu George

Reviewer: AudiophileHeaven
Pros: Build Quality, Bass Speed, Treble Extension, PRaT / ADSR (Textures) when properly amplified, Comfort after they loosen up a bit, Detail, Clarity, Resolving Ability, Scales well with a good AMP, Natural Soundstage, Good Dynamics, Quite Engaging, Effortless Sound, Good cables included in the package
Cons: Not very portable and mainly intended for desktop usage, AMP picky and pretty hard to drive, very open-back
Sennheiser HD660S - Changing The Status Quo

Sennheiser HD600 and HD650 have both been emblems of quality in the audiophile world for almost ten years now, with HD660S coming to change that and bring a new headphone to the table. We're having a look into what it brings to the table, and into how it changes the headphones that are already loved by thousand of people from all over the world.


Sennheiser is probably a name every single music lover has heard about once, be it about their epic headphones, or about their recording instruments, especially their high quality microphones which are iconic now, for both music lovers, film makers, and for basically anyone in need of high-quality audio manipulation equipment. Sennheiser has already proven themselves to be a very trustable and user-friendly company, when they actually helped hundreds of people with their IE800, me being one of them, back when there were some issues with an early batch of IE800. With outstanding support and quality behind, Sennheiser products are generally easy to recommend to anyone, from the most avid music lover to those just starting on this magnifique journey.

It should be noted that I have absolutely no affiliation with Sennheiser, I am not receiving any incentive for this review or to sweeten things out. This review is not sponsored nor has been paid for by Sennheiser or anyone else. I'd like to thank Sennheiser for providing the sample for this review. The sample was provided along with Sennheiser's request for an honest and unbiased review. This review will be as objective as it is humanly possible, and it reflects my personal experience with Sennheiser HD660S. Every opinion expressed is mine and I stand by it, the purpose of this review is to help those interested in Sennheiser HD660S find their next music companion.

About me


First things first, let's get the packaging out of the way:

Sennheiser has always been quite glamorous with their packaging, from the HD380Pro I've owned about ten years ago, which came in one of the most beautiful packages I had a chance to see back then, up to their latest flagships, like this HD660S, which comes in a pretty complicated, but well thought package.

The main headphone box is made out of cardboard, and inside it you find a hard carrying case, if we can name it a carrying case, but it is more of a storage case. It is quite sturdy and it has a large amount of sponge inside to protect HD660 during transport and storage. The unboxing experience is quite nice, and you know you're dealing with a high end product when you purchase HD660S.

When it comes to the box contents, it is quite a bit better than I was expecting. You're getting HD660S, two cables, one terminated in a 6.3mm Single Ended Jack, and one terminated in a 4.4mm Balanced Jack. Sennheiser throws in a 6.3mm to 3.5mm adapter so you can use HD660S with most Players and sources, since both 4.4mm and 6.3mm are pretty exotic connectors at this moment.

There are manuals included with HD660S, thing which is quite lovely, and there is a little bonus 7-day bonus membership to a live concert site in the package.

The package doesn't include spare pads, or anything else, thing which might be a little disappointing, but the headphones come with everything you might require to fully enjoy them, and while we'd like to see at least one set of spare pads included with headphones in this price range, we can't complain about the package not being good enough.

What to look in when purchasing a high-end Headphone

Technical Specifications

Impedance - 150 Ω

Connector - 6.35 mm / 4.4 mm Pentaconn

Frequency response - 10 – 41,000 Hz (-10 dB)

Sound pressure level (SPL) - 104 dB at 1V 1kHz

Ear coupling - Over-ear (circumaural)

THD + N, total harmonic distortion and noise - < 0,04% (1 kHz, 100 dB)

Transducer principle (headphones) - Dynamic, open

Weight w/o cable - Approx. 260 g (without cable)

Build Quality/Aesthetics/Fit/Comfort

Starting with the build quality, HD660S is quite sturdy. One thing that really feels different when switching from Audeze LCD-MX4 to HD660S is the weight, HD660S is incredibly light, and even in comparison to something lighter, like Ultrasone Signature DXP / Signature Studio, they still feel quite a bit lighter.

Of course, this may not be best for all users since weight can add to the feeling of rigidness and reliability of a product, but still, it is impressive that Sennheiser managed to pull this off.

The uild quality includes mostly plastic, Sennheiser HD660S has some other materials that are not plastic, like for example the drivers, or the grills on the drivers / the padding, but this shouldn't be taken negatively, since there are plastics that can resist to extreme amounts of weight and pressure.

The aesthetics will be somewhat familiar to those who've seen a HD600 or HD650 before, as HD660S is quite similar in shape and design to those two. The main largest difference is that HD660S comes in the color black, and they have a matte surface, compared to the glossier HD650. HD66S is also black compared to the textured / colored variants of HD650 and HD600. The headphone is mainly defined by a large pad, with the driver in the center. The driver is protected by metallic grills, and it is quite large in size. The Sennheiser branding can easily be noticed all over the headphone, and this is quite welcome as their logo looks pretty sleek and sexy.

The driver has multiple layers of protection, but none of them seals the driver, as Sennheiser HD660S is an open headphone to the largest extent of it.

On this note, they are more open than either Audeze LCD-MX4 or Beyerdynamic Amiron which we reviewed before, HD660S leak quite a bit more sound and they isolate less than either of those two models.

The fit and comfort depends a little on how much time you give them. Out of the box, HD660S is quite tight and their fit is quite tight on the head, and while this might mean that they won't be fully comfortable for any head, it also means that they are pretty secured in place, you can bang your head quite a bit without them falling off your head, and you can do quite a bit of movement without fear that they'll fall. It is a known fact that HD6XX series headphones become quite a bit more comfortable with some wear, and we are able to confirm this, after wearing them for a few weeks they can get so comfortable that you may forget they are on your head. The earpad width is large enough for most ears, but it is quite a bit smaller than that of Amiron, for example, which is quite a bit larger. The earpad material, on the other hand, is really nice to the touch, pretty soft, but with a texture, allowing for an excellent overall comfort.

Sound Quality

The sonic performances of HD660S are quite impressive, as much as you'd expect from a successor of the highly acclaimed HD650 and HD600.

We feel like we should talk a bit about HD600 and HD650 and their sonic quality before proceeding with our HD660S sound analysis as most readers are probably quite curious about those two as well.

HD600 s the first of the three to have entered the market. Sonically speaking, it is the quickest, the fastest, the most accurate, and the most neutral. Its sound can be described as mostly neutral, musical, very very natural in the midrange, with a well-expressed treble, and just a very impressive overall sonic performance. There's very little to fault when it comes to HD600, but if there is something that some listeners wanted, that was more bass, and a thicker sound.

This is where HD650 entered the scene. HD650 is thicker, slower, smoother, leaner and considerably darker than HD600. Some people felt that HD650 answered their question like the song of an Angel, while others, especially those who were quite comfortable with HD600, felt that HD650 wasn't quite the improvement they expected as the sound felt a bit veiled and maybe a touch too dark in comparison.

For rock and metal listeners, along with most Electronic music fans, HD600 was still the top choice, they kept that quick PRaT and texture, along with their incredible detail and naturalness, and most importantly, the energy. HD650, on the other hand, won over those who find a delight in classical music, in orchestral, and in Jazz, with their smoother and leaner nature, as HD650 is one-of-a-kind headphone if you're looking to relax and to feel euphoric with your music.

The basic signature can be described as slightly dark compared to something fully neutral, but not overly so. It is warmer and more musical than something like HD600, it has more detail, more clarity, and it has better definition than HD650. It would be more of a headphone to satisfy both the users who love HD600 and those who love HD650, bringing forward the best of both worlds.

Starting with the bass of HD660S, it is a clear, clean and snappy bass. It has excellent speed and clarity, but it is not a basshead oriented bass. It has enough speed to make you tap your feet, it has enough body to give music an interesting presentation, but it isn't quite a lot in amount, being quite close to what would be considered a neutral bass. This is not something negative in any way, and in fact, it could be considered a strong point of HD660S, but it should be noted that if you're craving large amounts of bass, there are headphones that provide much higher amounts of bass.

The midrange of HD660S is quite clear, well expressed and natural. In fact, this has been something quite noticeable about Sennheiser headphones in general, and especially of this series, but they manage to get a really true-to-life tone for their midrange, thing which is quite lovely for any kind of music you'll be playing from them. Rich is a good way to talk about the midrange of HD660S, and even if you prefer a presentation that is slightly different, trying HD660S at least once might open your eyes to a new signature.

When it comes to its treble, HD660S is quite energetic and bright. This is not a negative point, since brightness in music is quite welcome, especially with rock, metal, pop and electronic. There is a little bit of treble grain, especially if you're comparing it to something with much less grain, like LCD-MX4, but then, if you're a fan of rock and pop and metal music, you're almost surely going to be enhancing the treble response with a good few dB's anyway via your favorite EQ.

We'll talk a bit more about pairing in the pairing section of HD660S, but it should be noted that it eats quite a bit of power, and even if it is easier to drive than its bigger brothers, it still will play much better from a desktop source, most portable sources, even the best digital music players, being only borderline enough to pull the same results as a desktop source. For example, with Hiby R6, you might keep it at 100% volume, without any EQ engaged, and still feel that they aren't quite loud enough. It still is easier to deal with than its bigger brothers, who are even more picky when it comes to their sources, and both HD650 and HD600 require quite a bit more power than HD660S.


While we don't know Sennheiser's input in Equalizing their headphones, we always provide those were we're able to. The main thing that we felt would be welcome to add to Sennheiser HD660S was a touch more bass (and a lot more bass for certain music types), along with more treble. Happily, the Sennheiser driver technology is quite good and it can be equalized quite a bit without losing definition and without introducing distortion.

The main EQ profiles we'd apply on HD660S varied with the source used, and they usually have been like this:

31Hz + 6.3dB
62Hz + 3.5dB
16kHz + 7dB

Everything else is left at default, the master is usually brought at least at -7dB to avoid clipping.

This is pretty much it, but this will vary a lot with different sources since you can get a widely different signature form HD660S from different sources, so this profile won't be quite that universal in the end.


The soundstage of HD660S is aided by the fact that they are fully open, but the soundstage itself is not quite as large as other open back headphones. It is rather, a more intimate soundstage, with a great exposition of detail and layering but without going very wide. The depth is fairly good.

The main thing that defines HD660S when it comes to its soundstage would be called naturalness. While it isn't quite that impressive or that extended, like Sennheiser does with their HD800 series (the widest sounding headphone), the HD600 series has a naturalness that surely works well, especially for music like Jazz, acoustic and even for Rock and Metal, where the wall-of-sound is achieved fairly well.

The instrument separation and layering is fairly good, and on par with other headphones priced similarly. Sennheiser's own IE800 is where we consider the best layering and instrument separation to be, so once again, Sennheiser managed to do some amazing things with other models. The trick is that HD660S doesn't try to be quite precisely the same thing as IE800 or HD800, instead continuing where HD650 and HD600 left off. Compared, HD600 sounds similarly wide and deep, while HD650 feels more intimate and less wide than HD660S.


The ADSR and PRaT (Texturization) of Sennheiser HD660S are quite good. The sound is generally snappy and quick, the transient response is fairly well. Since there are lots of precise measurements online, you can get a pretty good idea of how things are, but we'd like to note this, the transient response can change quite a big with the source, so it can go from fairly natural, kinda like HD650, to pretty quick and textured, like HD600. They tend to sound a little more refined than HD600, where they bring a little refinement over HD600's textures, but HD660S usually stays ahead of HD650 in terms of speed and texture definition.

Portable Usage

Now, HD660S is not a portable headphone. Even trying to consider them to be one is wrong. We did take them on a little adventure in the Izvor Park in Bucharest to test this, but they still are not quite that portable, they would probably fit best with your favorite desktop and armchair setup.

The main fct that stops them from being portable is that they have a hunger for power, being hard to drive and picky about the source, they come with long cables, and it can be somewhat complicated to find good replacement cables that are shorter, and their cables are not as easy to tuck away as those of LCD-MX4, and HD660S is fully open.

Where even Audeze LCD-MX4 is somewhat portable, and we were able to use it portable, HD660S is probably best kept inside and used there. Their pairing with desktop-class AMPs and DACs means that you're going to get the best results as such, and besides iBasso DX200 and DX150 paired with AMP4S and driving HD660S in 4.4mm balanced mode, we'd probably stay with something that is also desktop-classed as AMP, like iFi iDSD Micro Black Label, Burson Play, or Sennheiser HDVX series of amplifiers.

The comfort while being worn for long periods of time is very good, and with HD660S, you can walk and even to some heavier exercise without damaging them, or without feeling any fatigue, and it surely is possible to wear them for prolonged periods of time without much issue, but you should mind the cable, which is quite long. There are no microphonics, and no other issues we were able to notice, but HD660S does not isolate from the outside noise (at all), and most music being played through them will also be heard outside, so they are quite open in this aspect.


Sennheiser HD660S vs Sennheiser HD600 - Sennheiser vs Sennheiser, HD660S feels like a more natural, more refined, more mature, and better looking version of the original HD600. It is easier to drive the HD660S, and it will fit in most situations where HD600 won't, but HD600 still has the crown of being the fastest of the HD6XX series from Sennheiser.

Sennheiser HD660S vs Sennheiser HD650 - Here, the difference is much higher and much more in the advantage of HD660S, as HD650 doesn't feel quite as refined, nor as well expressed or as detailed, and as explained in the sonic performance, HD650 feels slightly veiled, as reported by some users. While this veil isn't quite something to notice if you're listening just to HD650, if you add \HD660S, you can totally notice how everything becomes clearer, more vivid, more dynamic, more expanded in every direction. The smothness and laid-back character of HD650 are something that HD660S doesn't have quite as much, so if you need the absolutely smoothest and most relaxing headphone of the HD6XX trio, then HD650 is still the king in that aspect, but if you're looking for the most versatile one, HD660S is pretty much the best choice.

Sennheiser HD660S vs Beyerdynamic Amiron - This is a much more fun comparison to make as Amiron is from Beyerdynamic, and here the differences are much easier to notice. Let's begin with the package, HD660S comes with less carrying box, while Amiron comes with a carrying box. The cables included are better on HD660S since they come with a balanced cable, which is terminated in the 4.4 Jack, along with a single ended cable. The quality of the cables themselves is pretty similar though. The build quality is quite similar between them, although Amiron has metallic parts in the headband, while HD660S is mostly made of plastic in those areas. Amiron has a much larger earpad, it sits much more comfortable on larger heads and larger ears. This also means that Amiron will fall easier and offers less clamping force, so your choice should be made according to your needs and preferences. The sonic performance is quite different, along with their tuning. HD660S sounds much more natural, thicker, has a leaner and more laid-back sound, where Amiron has a brighter top end, a deeper sound with a larger soundstage, and with a darker midrange. This makes Amiron sound a bit more mature, and it would come off as more suited for music such as Jazz, Classical, Orchestral, but also pop and certain types of metal, where HD660S comes off as better suited for Metal, Rock, Pop and other types of music where a more natural midrange is more important. The other aspect one should consider is the revealing abilities, there is a difference in how Amiron and HD660S reveal the textures, as Amiron is smoother in the textures, despite their brighter treble, they are smooth and lean, where HD660S is more grainy in the treble, and has a tad more texture across the whole range, they tend to provide a more textured sound. Amiron is quite soft in comparison, soft, velvety and relaxing, but with a sparkly treble and a wide/deep sound, where HD660S feels more forward, more textured and more personal. From the two, HD660S feels more like a punk/pop headphone, where Amiron feels more mature and like a better Jazz / relaxing headphone that still keeps a lot of excitement with their treble spark.

Sennheiser HD660S vs Ultrasone Signature DXP - Ultrasone Signature DXP is the last headphone we're going for in this comparison, and they come at a price point rather similar to HD660S. Starting with the package, HD660S comes in a leather carrying case, which doesn't quite get our preference over Sennheiser's storage case because it isn't much more practical since the leather is quite sensitive, and you'll still need supplementary protection. The accessories included are comparable, since Signature DXP also comes with two cables, but both are single ended. The largest difference in build quality is that Ultrasone Signature DXP is a closed-back headphone, which is fairly easy to drive, offering an entirely different set of usage scenarios when compared to HD660S. The build quality is quite great on both, both being mostly plastic, but Signature DXP featuring something that looks like glass insertions in the headphone cups. The closed-back design is probably less sensitive to the environment. The comfort is better on HD660S as they have larger and deeper ear pads, where Signature DXP feels smaller on the ears, with slightly shallower ear pads. The biggest difference here is the pad type, as HD660S has a velour-like material on the earpads, where Signature DXP has leather on the pads. The sonic performance is quite different, Signature DXP featuring much more bass, more tactile, more dynamic, more solid, quicker and better textured, a similarly natural midrange performance, and a similar treble performance. In the midrange and the treble, DXP feels leaner and provides a softer presentation, where HD660S might feel slightly more enhanced in the treble, with a bit more bite. The biggest difference between the two there will be in the detail and soundstage though. Signature DXP has a huge soundstage, making HD660S feel more intimate and more personal, where DXP feels quite open and wide. On this note, the detail on HD660S is better, and by a good margin. Simply put, they are more revealing, offering a more transparent window to the music that is being played, where Signature DXP feels more playful and colored, but not more detailed than HD660S. The usage scenarios between the two will be quite different, but if you're a basshead, then Signature DXP is a much better option than HD660S, which will surely appeal more to those looking for a headphone with a linear / neutral bass. The biggest thing one needs to keep in mind is that both will play music very well, and will work similarly well with similar music, but if you prefer a wider experience, then Signature DXP might be the better choice, while if you're looking for a more detailed, more mature and more personal experience, then HD660S will be the better choice. We feel like they are made for different people, but still a good to know comparison between the two.

Recommended Pairings

Sennheiser HD660S is quite picky with its AMP and will favor desktop and powerful sources over portable ones. In fact, they are so hard to drive that very few of our portables were able to make them sound loud enough for a satisfying experience, being pretty similar to Amiron in this aspect, although Amiron was quite a bit easier to drive by direct comparison, HD660S being the hardest to drive headphone we reviewed to date. They work very well in 4.4mm balanced mode as well as single ended mode, provided you're feeding them enough power.

Sennheiser HD660S + iBasso DX200 / DX150 (AMP5) - This is a pretty handy combination, because AMP5 has quite a bit of power and it is able to drive HD660S quite well, providing the versatility of DX200 and DX150 in a pretty portable package. This whole combo is not very inexpensive, but it surely can provide a lot of fun. The sound is generally well balanced, HD660S sounds dynamic, it has a good soundstage and instrument separation, and they have a pretty quick and clean overall sound. The transient response is fairly natural, and textures feel natural.

Sennheiser HD660S + iBasso DX200 / DX150 (AMP4s) - This combo has the same versatility as DX200 and DX150 combined with AMP5, but they have a 4.4mm balanced connector. The balanced connector isn't quite the reason why they'd sound different, but we consider that the difference in sonics lies within the different amplifier architecture, the improved capacitors and the different power configuration. Basically, we don't endorse the concept that Balanced is always better than Single ended, but in this situation we like the 4.4mm native balanced connector quite a bit, and the sound is more vivid, more dynamic and has a touch more detail than AMP5, which is still quite great. The AMP4S variation from iBasso is also slightly warmer, more effortless and slightly leaner, so this should be taken into account. Since HD660S may need more bass, this is a welcome improvement.

Sennheiser HD660S + iFi iDSD Black Label Micro - iDSD Micro Black Label is quite the champ when it comes to driving HD660S. The brighter treble spike of iDSD Micro BL adds to the airiness and treble bite of HD660S, thing which is quite welcome and helps bring more life into them, and the strong bass, and especially the X-Bass feature will help those looking for a bassier sound with HD660S. It should be noted that iDSD Micro BL requires a separate source, as it is a DAC/AMP, so you should factor in the cost of another device when planning the listening setup.

Sennheiser HD660S + Burson Play - Burson Play is a great little device created by Burson, a rather loved and well-known company. Play is most an Audiophile-Grade, modular, integrable DAC/AMP for desktop, and especially for desktop PCs. Its power is quite fit for HD660S, and it is a good way to prove that HD660S can sound pretty sweet on a proper desktop-class setup. The beauty of Burson Play comes from the fact that you can integrate it in your PC setup, so you can plug HD660S, or any other audiophile-grade headphone, directly in your battle station, and just enjoy audiophile-grade sound without having more devices on your desk or table. The sonic performance is quite variable with Burson Play, as it allows the user to change its OP-AMPs, but the most basic configuration offers a warm and smooth sound, with a pretty good soundstage, which pairs well with HD660S.

Value and Conclusion

At the end of the day, you're probably wonder where is the HD6XX line headed towards, and we feel it is headed towards greatness. Sennheiser has shown great respect for their customers, and they have shown improvements organic with their customers demands when releasing the HD660S, which is a more versatile headphone than both the very clean, but slightly clinical sounding HD600, and the too lean and too smooth HD650. They simply made a headphone better than either, one that not only bests both, but will also be hard to beat for a long time to come, as its predecessors were iconic headphones for many years so far.

First, we need to consider the value HD660S has at the moment of writing this review. They sell for around 500 USD in most parts of the world, which places them in direct competition with pretty amazing headphones like Beyerdynamic Amiron / DT1990Pro and Ultrasone Signature DXP / Signature Studio.

The package contents of Sennheiser HD660S package are quite good, they come with two cables, one of which is terminated in a 4.4mm balanced connection, currently our favorite type of balanced connector. They also come with a 6.3 to 3.5 mm adapter, so you can plug them in virtually anything, but they eat quite a bit of power, so in the end they will require proper desktop-class amplification for most users. They don't come with spare pads, or a spare headband, but the quality of the earpads and the headband is quite good and we don't really expect anyone to require spares soon.

Where the original HD600 and the HD650 were headphones that stood their ground for years, we feel that the new HD660S will also be a headphone to shake the world of music lovers for a long while. The sonic signature of Hd660S takes the best of both worlds, the best of HD600's sound, and the best of HD650's sound, and makes a headphone better than both, with the speed and resolution of HD600, the naturalness and effortless sonic reproduction of HD650, with the more romantic bass of HD650 and with the sparklier treble and airier / wider sound of HD600. Basically, it bests both, and becomes a whole new headphone, more versatile, more portable, more comfortable and better built than either the beloved HD600, or the romantic and relaxing HD650.

If you're looking for an open-back over-the-ear headphone in this price range, there are quite a few options available to you,m and even Beyerdynamic now has wireless versions of their Amiron, so you really have where to choose from, but if you want to experience a headphone with Sennheiser's pedigree, and if you never heard the HD600 or the HD650, then now is your chance for an even better experience, with the newly released HD660S. Also, if you already own either HD600 or HD650, and if you're looking to upgrade, now is your chance, as we feel that most people will find HD660S the more versatile, better sounding headphone than either of its beloved predecessors.

I hope my review is helpful to you!

Stay safe and remember to always have fun while listening to music!

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Great review, purchased 3 days ago & they are brilliant !
Dobrescu George
Dobrescu George
@Rob49 - Really happy to have helped! :)
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We wanted to share a related comparison video (not a review) of the HD 660S, HD 650, and HD 600 to help visualize some key differences and similarities within the 600 family. For instance the mesh baffle and vented magnets of the 660S, etc.

This review is a classic and has helped many in their quest for impressions and feedback. Even years later it's still exemplary!


Reviewer at Soundnews
Pros: High comfort, solid construction and durability
- Good tonal balance, rich and full of substance
- Balanced frequency response, nothing stands out too much
- Airy presentation that leads to good holography
- Let the veil be lifted! Good riddance to muddy and veiled sound
- High level of detail retrieval for the price category it belongs
- Back in black matte paint
Cons: Shy sub-base
- Slight distortion on sub and mid-bass
- Medium size soundstage
Dear friends,

If you wonder why we have done so many reviews of Sennhseiser gear and headphones and never of the legendary HD 600 or HD 650, you have to know that we wonder the same.

All of those who followed the wonderful journey into the world of headphones inevitably bumped into and listened to the Sennheiser HD 600 or HD 650 at least once. They quickly became so popular and widespread that we considered a dedicated review unnecessary.

Sennheiser recently announced the true successor of the HD 6xx line, the new HD660S.

As a sign of repentance, we bring out the fresh review of this new model, and at the end of this article we will compare it with the predecessors HD 600 and HD650, which we’ve been using for many years now.

Built and appearance

The design is very similar to the HD 6xx line. They’re built almost completely from hard matte black plastic that won’t be a fingerprint magnet anymore. The exterior grill is still metal, with an embossed Sennheiser logo on one side.

The earpads are a bit deeper and softer, which allows for longer listening sessions.

Fresh out of the box, the clamping force of the headphones is a bit stronger, but most probably in time it will loosen at the level of the HD 600/HD 650.

What more can I say? We have the same built and design that we are familiar with for more than 26 years.

But why would you want to change something that stood the test of time with ease?

Besides the standard 3m long cable terminated in a 6.35mm jack, there is a 6.35mm to 3.5mm adapter, and an additional balanced cable terminated in a 4.4mm Pentaconn jack which is being used more and more by the leading industry. We’ve already seen this balanced connection on the new Sennheiser HDV 820 and on many of Sony’s gear and headphones. I hope others will adopt this new standard soon.

HD 660S Driver

What is truly new is the driver.

The impedance is lowered to 150 Ohm. Sennheiser are marketing these headphones as compatible with portable devices (smartphones and music players).

Honestly, I can’t imagine a way to get good dynamics or a holographic experience out of a smartphone.

Maybe the halved impedance would lead you to believe that the sensitivity is much better, but you’d be wrong. In reality the headphones are 2dB more efficient that the predecessors, so in real world tests they sound just a little bit louder.

In conclusion, I don’t recommend you to plug them directly into a smartphone, but if you add a portable amplifier to the mix, things will get radically better.

HD 600 driver

Looking through the technical specifications of the high end Sennheiser models, I see an obvious similitude to the HD 700. I even have a theory according to which Sennheiser might have had a HD 700 driver excess stock and “poof!”, the new HD 660S was born. But this is just a theory that I cannot fully prove.

I observed similarities in the dampening material behind the diaphragm and in the metal mesh behind the diaphragm. Its shape also resembles that of the HD 700.

Anyway, what is really important is that sonically HD 660S is quite different from the HD 700, which for me is a good thing.

But now let’s get to the most interesting part.

Sound quality

Well, every morning this week I lifted my mood with the album Cei Ce Ne-au Dat Nume by the Romanian rock band Phoenix.

Electrocord is not Chesky or Reference Recordings, I know, but I consider it a quality recording for 1972, from which nowadays many could learn a thing or two.

From the first notes of Negru Voda – Baladă, even though the song becomes complex very quickly, I was impressed by the precise positioning of each acoustic instrument in a distinct place.

The electric guitar, the bass guitar, the violin and the drums seem not touching each other, they don’t compete for the same space, there isn’t a struggle for which to come forward and which to be left in the background. The headphone never sound hectic or crowded.

When changing the HD 650 with the HD 660S after a non-stop 2 hour listening to the HD 650, I can feel that the veil has lifted, the mid-bass stops rattling and clipping and the harmonic distortion feels lower.

The veil that I was talking about is much more present on the old versions of HD 650 and HD 600. The new HD 650 and HD600 versions are better in the respect. Even though the manufacturer doesn’t acknowledge any changes, the changes are there, the dampening material behind the diaphragm has been modified and the new HD 650 sound cleaner and less distorted.

This problem is no longer present on the HD 660S and this is probably one of the best improvements of the new model.

In any case, if you like a slower smoother presentation, I think you’d better keep the HD 650 because it does this better.

Listening to local rock music, I noticed a much lower mid-bass and voice distortion and a quicker decay of notes, everything seems quicker and instantaneous.

Despite their well known qualities, both HD 650 and HD 600 have never been praised for their quick transients. Their sound is soft and slow, lacking in speed even paired with a fast solid state amplifier.

In this regard, the HD 660S is two levels above.

Only now I can say that HD 660S has enough speed and impact that it can rival the mid-level planar magnetic headphones.

Every cause has an effect, so the quick transients unearth hidden micro-details that the HD 650 is not capable of rendering.

Yes, the HD 660S has more resolution and clarity. It still cannot touch the HD 800 in this regard, but I’m happy that things evolved to this high level for the modest price compared to other models that we’ve reviewed.

Another positive effect is the much more precise timing.

If you carefully listen to fast electronic music on the HD 650 or HD 600, you can hear a micro-echo, like an unpleasant resonance, maybe and effect of the over-damping behind the diaphragm (again just a guess). This effect damages the delimitation of notes and each individual sound end up not being sufficiently well defined.

The HD 660 S have a more precise timing, giving the sensation that musical notes are fuller and more precise, better rounded, with a clear definition.

In the long term this could induce listening fatigue, but even so, the timing is more correct and true to reality.

Listening to the song Strunga from the album Mugur de Fluier, I notice the impressive soundstage that is well expanded in all directions.

I now realize that none of the models in the HD 6xx series, including HD 660 S, offers a convincingly large and expanded enough soundstage. The soundstage is somewhat compact and intimate, but surely not claustrophobic. I would categorize it as a medium-sized soundstage, not too large (like HD 800), but also not so close as to make me feel on the stage with the band.

In this respect, all 3 headphones are almost identical.

The depth of the soundstage is pretty good. With my eyes closed, I can walk myself through the quality recordings. On well recorded material I could never hear the sound becoming too crowded.

Interestingly, despite the fact that I don’t feel the notes flying a few meters from me, I can clearly hear the angle at which they are emitted, a good sign that the holography is in effect.

I want to mention that, from the 3 models, the HD 650 scaled the worst in this regard. I can feel myself distracted by the micro-echo and harmonic distortion in the bass and mid frequencies, and the slight veil over the notes also impairs the listening experience. By comparison, the HD 600 seems to bring a fresh breath of air and doesn’t have as many shortcomings.

Anyway, the superior precision of the HD 660 S is in effect, the model offers the most credible three-dimensional holography of the 3 models.

Frequency response

Even though the majority of the HD 650 users might say that it excels in the bass department, I am of a different opinion.

The mid-bass clearly stands out and always attracts attention on itself, continuously screaming “look a me!”.

But before we deal with the mid-bass, the lower bass is problematic, not only on the entire HD 6xx line, but also on all the open dynamic driver headphones I know.

Listening to Infected Mushroom – Return to the Sauce I can sincerely attest that it’s an extraordinary experience!

The songs Manipulator and Groove Attack put any diaphragm to the test in terms of speed, impact and bass depth.

The HD 660 S vibrates well enough on bass and the mid-bass is prominent, like with it’s predecessors.

The sub-bass though sometimes leaves to be desired. The sensible use of an equalizer can solve the problem, but I don’t recommend adding more than 4-5dB in this area because the driver can start clipping and distorting when the volume is pushed above a certain point.

To make myself fully understood, the headphone doesn’t have a severe sub-bass deficiency, some people categorizing it as bass-heavy, but you cannot understand what real sub-bass is until you give a listen to some well amplified high-end planar magnetic headphones.

I sincerely think this is the biggest drawback of these headphones, maybe the only drawback that caught my attention on the HD 660 S as well as on all other dynamic driver Sennheiser models: the sub-bass starts to distort beyond 95dB of volume. I think this problem is not even present for an untrained ear. My father still claims the HD 650 sounds crystal clear, so it depends on the listener.

The mids, oh, the sweet Sennheiser mids.

Yes, they are still here, they haven’t left. Probably the biggest virtue of these headphones is the very present mids that seem to infuse joy directly into your central nervous system.

There aren’t many headphones in this world that gave me goose bumps when listening to violins. The headphones in the HD 6xx line have always known how to do this. I cannot explain how, but they simply excel on mids and, yes, they highlight them a bit too.

What the heck! Midrange frequencies are 90% of the sounds we hear in our everyday life, so if you cannot render them properly, everything is lost.

The HD 650 and HD 600 have always been categorized as being more musical than the other models and this has been achieved through the good reproduction of the midrange.

The HD 660 S doesn’t stray from this formula.

There is although a little difference, the mids aren’t so forward as with the HD 650. The HD 660S places them only slightly forward, it doesn’t push them that hard, so it sounds more balanced, less melancholic and musical. It’s not a big difference, but is noticeable.

If you like the overly seductive character of the HD 650, maybe you should stick to it, but if you desire a more sincere sound, the HD 660 S does it much better. It doesn’t try to hide anything but also it doesn’t overemphasize anything.

In my opinion, the HD 660 S is the true reference in the HD 6xx line, with the HD 600 coming second and the HD 650 coming last, with a more colored sound.

When it comes to sound density, all 3 models are effortless. The sensation of a meaty sound, full of substance and density is best present on the HD 650 and HD 660 S.

Another cause and effect: the dryness of sound is never present. I detest dry artificial sound and luckily the HD 660 S doesn’t have this symptom.

Listening to Pink Martini and Lorena McKennit it becomes crystal clear that this kind of music was created for the HD 660 S. The warm voice simply touches your soul and the instruments become a game of the imagination.

If you listen to acoustic music, jazz or classical, the HD 660 S becomes an obvious recommendation.

The high frequencies are differently rendered on each of the 3 models.

I think that the HD 660 S handles them the best, followed by the HD 600, with the HD 650 being the last in this respect.

It’s easy to confuse clear and well defined highs with sibilant highs – one of the biggest mistakes that I see coming from some online publications. Some consider the HD 660 S to be sibilant but I tend to differ completely, I consider it having clean highs, without highlighted extremities.

The highs presented themselves clean and airy, with a good balance between technical show-off and naturalness. The highs are somewhere in the middle, not overly natural, but also not overly bright and sibilant.

The HD 600 has less airy and less detailed highs than the HD 660 S, and the HD 650 has the most washed out highs of the 3 models, sometimes the high registers being placed in the background.

As a whole, the frequency response appear to be the most correct and complete on the HD 660 S. Yes, they’re a little more neutral from the midrange upward, but they never sound boring for their price category.

Talking about the price, the HD 660s being a new model, at the beginning of the life cycle, 500 EUR seems to be a reasonable price for what they are. The HD 650 had the same release price. At the moment the HD 650 can be bought for much less than the official original price, but let’s not forget that they’re 14 years old, it’s just normal to be much cheaper now.

If you already have the HD 650 or the HD 600, it’s hard for me to recommend something that is a bit better. You have to give them a listen and decide for yourselves.

If you don’t have any of the HD 6xx series models, then HD 660 S becomes a much easier recommendation. If you want the best model of the HD 6xx line, the HD 660 S is that model. If you’re on a tight budget and you want the best for your buck, then the HD 650 and HD 600 come on top as the best recommendation.


Like I told you, the HD 660 S is only 2dB more sensitive than the HD 650 and HD 600, so the halved impedance has very little impact on this.

Maybe Sennheiser would suggest that you can drive this headphone directly out of a smartphone, but I consider it an April’s Fools Day joke. This headphone need the cleanest possible amplification and, of course, a dedicated one. I’ve managed excellent results with an Audio GD Master 9, and also with a HeadAmp Pico Power, and even with the ordinary Pico. The headphone simply comes alive, the dynamics raises your pulse, it’s a completely different story with dedicated amplification.

Out of curiosity, I connected them to a dedicated mid-range DAP, FiiO X3 MKIII, on the single ended 3.5mm and it sounded more than decent. The headphone came alive even on the little X3. The high-end players would surely deliver even more performance.

The myth of amplifying the HD 660 S with a smartphone is now busted!


VS HD 650

The HD 650 loses the tonal balance quickly, overemphasizing the mid-bass and cutting out the high treble. The overly forward mids crowd the soundstage a little bit, the sound becomes a bit claustrophobic, especially on bad mastered music. The HD 650 sounds a bit slower and veiled, the definition of musical notes is less outlined and a micro-echo comes to the mix, together with a bothering distortion.

All of these disadvantages aren’t present on the HD 660 S.

VS HD 600

It’s funny that even if, from my understanding, the HD 660 S is meant to replace the HD 650, the HD 660 S resembles the HD 600 more than the HD 650.

The highs are similar on both models, with the HD 660 S winning in the very high registers, where you can still hear new details out of the background.

The mids again are very similar, with the HD 660 S being a bit cleaner and better defined. I can hear the string vibration and the decay of the vibration more clearly, the HD 600 masking these subtle details.

The bass is a different story. The HD 600 has a weaker bass and the HD 660 S has a deeper and faster bass. The bass is the only area where the harmonic distortion can become a problem, especially if you tweak the EQ too much. The HD 660 S scales much better in this regard, offering a lowered harmonic distortion in this area.


If you liked the HD 650 and the HD 600, it’s impossible not to like the new HD 660 S.

I’ve seen opinions according to which the HD 660 S is inferior to the HD 650 model, but I couldn’t figure out what they are based on.

Maybe if you like a more lazy, relaxed and less dynamic sound with less detail, then yes, the HD 650 can bring exactly this to the table.

The built quality is already well known for 26 years. The matte black color is welcome, as well as the 2 cables in the box. I hope other manufacturers will adopt the new Pentaconn 4.4mm balanced jack. I think this is the future of balanced connection; we’ll live and see.

The official price seems correct, exactly the price of the HD 650 14 years ago. If we factor in for inflation, they can be considered cheap for the current year.

In my opinion, the legend continues with HD 660 S, being a fierce contender in its price category.

Equipment used for review:
Sennheiser HD660S, HD650, HD600, Matrix Audio X-Sabre Pro, iFi iPurifier2, Audio-GD Master 9, Audeze LCD-4, FiiO X3 MKIII, HeadAmp Pico & Pico Power
Original review posted Here
The closest “affordable” upgrade to the 650 is the Focal Elear (Street price $670 and dropping). Still over 3 times the price of the 650 clone, the Massdrop 6xx. With the Focal Elear, if you’re lucky, and on a good day, you’ll get 5-10% improvement over the 650.
Great review Darku. Very well written and honest, displaying your feelings as well. Makes me love the 660s even more
Great review.


1000+ Head-Fier
Pros: Much easier to drive, amazing mids, standard and pentaconn balanced cables included, detachable cables
Cons: Building material not what price point would expect, doesn't scale as well as one would think, very stiff clamping force

This was a real surprise for me. A representative of Sennheiser reached out to me via head-fi asking me if I would be interesting in trying out their brand new and successor to their legendary HD650 headphone. Thank goodness this was in the form of private message because being overly excited at the opportunity to try something as new as the HD660 S most certainly raised my vocal octaves a few notches. So needlessly to say I happily accepted the offer.
What made this opportunity all the sweeter is that I’ve been having a few subscribers on my YouTube channel ask me for my thoughts on the new HD660 S and with each I’ve had to give a ‘I’ve yet to hear but I hope to soon response’, well thanks to the very kind gentleman from Sennheiser I can finally give my true opinion and thoughts on the proclaimed successor to one of the most legendary headphones ever to grace audiophilia.

A little about me
I would like to say that first and foremost I am NOT an “audiophile” but rather an audio enthusiast. I listen to music to enjoy it. Do I prefer a lossless source? Yes, of course. But I can still be very happy streaming from Pandora or even YouTube's “My Mix” playlist. I also prefer equipment that sounds the best to me personally regardless of what frequency response it has or rather or not it's “sonically accurate” and I always have and shall continue to encourage others to do the same.
I'm a 26 year old firefighter, for the City of Concord, North Carolina as well as the U.S. Army, North Carolina National Guard. The cliché of wanting to do this since I was born couldn't be more present with me. I've worked hard over the last several years to earn this position and now it's time for me to work even harder to keep it.
My interests/hobbies are fishing and relaxing to audio products and reviewing them to help other decide on what products would work for them. Few things make me as an audio enthusiast/reviewer feel more accomplished than when someone tells me that I helped them find the type of sound they've always been looking for.
Now, the sound signature I personally favor is a relaxing, warm and sensual sound that just drifts me away in the emotional experience of the music being performed. Yes, accuracy is still important but I will happily sacrifice some of that if I'm presented with a clean, warm sound that can wisp me away into an experience that makes me yearn for more.
My ideal signature is that of respectably forward mids and upper bass range with the bass being controlled but with some slight decay. I like my treble to have nice extension and detail reveal with a smooth roll off up top as to not become harsh in the least. Examples of products that have given me chills and keep giving me the yearning for more feels are the (in no particular order) Bowers & Wilkins P7, Oppo PM-1/2, Empire Ears Hermes VI & Zeus XIV, Audeze LCD-XC, Meze Headphones 99 Classics.
Equipment used at least some point during the review
-Sennheiser HDVD800
-PS Audio Digital Link III w/ Cullen Stage 4 Upgrade
-LG V20
-Playing Pandora, YouTube, and various lossless, FLAC, etc... music
-Misc. Equipment
-Source cleaner
-iFi Nano iUSB3.0
I am by no means sponsored by this company or any of its affiliates. Sennheiser was gracious enough to send me this product in exchange for a honest review. The following is my take on the product being reviewed. It is to be taken “with a grain of salt” per say and as I always tell people, it is YOUR opinion that matters. So regardless of my take or view on said product, I highly recommend you listen to it yourself and gauge your own opinion.

The Opening Experience


Why I feel so strongly about the initial unboxing experience
Please allow me to explain why I feel so strongly about the initial unboxing experience with a product. Maybe it’s due to my southern roots in the hills of eastern Kentucky, but I’ve always been raised under the pretense of when you introduce yourself to someone for the first time you present yourself with confidence, class, character, pride, and competence. You greet the other person with a true warm smile, eye contact and a firm handshake. Anything less or short implies to other person that you either don’t care about them, are too full of yourself, too busy to be bothered by the likes of them, or worse, just generally disrespectful.
As a consumer, I take this same belief to when I open a new product. Why? Because think about it this way. How else can a company introduce themselves to their customers? How do they present their products? Are they packaged with pride and presented in such a way that makes the listener eager to listen to them? Or maybe they’re just wrapped up and placed in an available space. How about the box itself? Is it bogged down with jargon that says look at this, look what I can do. I’m better than anything on the market and here’s why read this and check out that. Or, is the package clean, simplistic and classy? As if saying to the customer ‘Good day, pleasure to meet your acquaintance. Please give me a listen and allow me to show you what I can do and allow my actions to speak louder than my words.’
This is why I feel so strongly about the initial presentation of a product, and I feel it’s truly a shame more people don’t. But with all that aside, let’s discuss how this products introduced itself shall we?
I believe I received a very nice “handshake” with the HD660 S, as I did with its predecessor the HD650. Sennheiser includes a very nice cardboard box that is really just a smaller version of the HD800 S box minus the nice silky material and the stop strap. The box itself, like the HD800 S, is my EXACT want in a headphone box. The exterior si super simplistic as in all it shows is a picture of the product and its name. This, to me, shows the competency and confidence of an audio manufacture.By doing this I see it as them telling their customers listen to my product and let it speak for itself; let it SHOW you what it can do instead of my boggling down the box with fancy words and look-at-me’s so that I can pretend to be competent. And for a company as legendary as Sennheiser, who’s over 70 year experience shows, they’ve got more than enough competence. So in other words, before I even get into this box, I’m extremely excited to put it on my head and see what it can do.

But I digress for my audio geek out, as you open the box to the HD660 S you’re greeted with that very nice new headphone smell and a headphone that’s nice and centered in cut out foam to keep the HD660 S safe during travel. Sennheiser also included the brand new 4.4mm Pentaconn balanced connector in addition to the traditional 6.3mm audio jack. Unfortunately I was only able to use the standard connection because at the time of the writing of this review there’s nothing in my inventory that takes that connector size. Then lastly you’re given the warranty and instruction manual in a nice packaging, that unlike the HD800 S only shows the HD660 S and not both it and the original HD650.

So again I’ll state that the “handshake” given to me from Sennheiser was firm and filled with confidence, which is exactly what I’ve come to expect from Sennheiser.






So I’m already super hyped up to put these on my head and try them out. The only issue I really had with the original HD650 was that for its price I felt it to be quite cheap feeling. So did Sennheiser make improvements in the successor to that legendary headphone? Nope…. In fact I feel that they’ve made the HD660 S even cheaper than the HD650. Granted, I’m no engineer and they’re both made of plastic so for all I know they may be exactly the same but with a different paint job but from me picking both up and feeling them I feel like the HD660 S is a lesser quality product (solely in terms of its construction) than its predecessor HD650. But that comparison aside, these headphones are very lightweight which should definitely help the users overall experience.

Moving down the headband still features it very nice and firm size adjustments that leave no doubts to the user what the cups have moved to a different size, for each adjustment is made with a notable pop. On that note, this headphone has more adjustment sizes than I think anyone will need. Unless you’ve a really tiny head, you’re more than likely to find the optimum adjustment out of this headphone.

The padding in the new HD660 S feel very firm to the touch and offer a really nice level of both give and support. This may be because these are a brand new pair and my HD650 is, how should I say, well experienced, the padding looks to be a little bit deeper than my ol faithful HD650. This is a good thing because my ears no longer touch and rub against the cloth covering the drivers.

Lastly we’ve the DETACHABLE cable. The cable on the HD660 S is the same that’s on the original HD650. It’s a decent quality cable. It’s not shielded or looks anything fancy but it does its job very well. To my ears there’s no microphonics or interference that I’ve ever heard during my time with either units.

So my thoughts on the build quality of the Sennheiser HD660 S is that it’s acceptable. Like in the HD650 I believe there’s a lot to be improved on considering these are $500, but I’ve also reviewed worse.





So they didn’t improve the build quality of the HD660 S over that of its predecessor so how about its comfort? Ummm, depends on what you like really. Where as the HD650 felt like “sand conforming to your hand” as I said in my HD650 review, the HD660 S is like the bear hug a relative gives you after not seeing you in a really long time. It’s not that it hurts, but it’s quite tight, and as a personal opinion, it’s way too much so for my preference so I PERSONALLY (so do at YOUR OWN RISK) bent the metal band on each side of the ear pads just slightly and it made it, to me, considerably better.

Getting back on track, Sennheiser made very slight changes to the padding making it both thicker and denser. You can really only tell a difference if you’ve had a HD650 for a considerable amount of time or had a pair on you next to the HD660 S and could sit them next to each other. But I feel that this was a great improvement over the HD650 because on the HD650 my ears would just slightly rub the cloth mesh covering the drivers but now I think that maybe the very tip of my ears do slightly but I only notice that when I’m consciously trying to figure out if they’re touching or not.

During my time with them, post the slight bending, I had no issues with having them on my head and listening to them for several hours without any fatigue. They stayed firm in place regardless of my lying position and truthfully, I really enjoyed how they felt on my head (again post bending [which again I stress to do at your own risk]). Perhaps time will break these into the forming sand the HD650 is to me but even if it doesn’t, what’s so wrong with the loving brace of a relative who really misses you?


Oh ho, here we are. Time to find out rather or not the HD660 S can uphold the claim of it being a successor to the legendary HD650, even stating that “the legend continues” right on the front of the box. At first listen before A/Bing the two side by side I originally couldn’t tell much of a difference just going off memory and thought, ‘ok sweet, they’ve just made a HD650 that’s more sensitive and updated its looks a little bit’. But immediately after popping in ol faithful the differences immediately become apparent.

The new HD660 S takes a different direction from the HD650’s smooth and calm presentation and favors a more direct and in your face one. The new HD660 S makes the mids and upper bass much more forward which in turns heightens vocals and acoustic instruments presence in the audio. This in and of itself is actually a really comforting sound and a very relaxing on as well, at least to me personally for most of you reading my reviews have a pretty good understanding on my musical preferences, but it’s the fact that Sennheiser directly considers this an upgrade from the HD650 and thus will discontinue production is what’s so off putting. If you can, listen to the song “Colour to the Moon” by Allan Taylor. There’s several songs that will pinpoint the differences between the two headphones but this song, to me, is the best I’ve personally found.

The comfortable soundstage and detailed imaging hasn’t changed much. This was kinda difficult to determine because the forwardness of the mids and upper bass makes the soundstage sound a bit closer than the original HD650 did but when playing normal classical/instrumental music, at least me personally, I can’t hear a notable difference between the two in the soundstage and detail side of things.

Imaging is also a really interesting one. If you were to ask me which of the two would be the “best” (I hate that word because I firmly believe there’s no such thing) for overall imaging I would side with ol faithful. The HD650 just plays so beautifully with pretty much every type of music that it’s ability to show the listener what they’re listening to is amazing. However, if you’re listening to a piece with a lot of acoustic presence, or even harder rock/metal, I think you’ll like the new HD660 S better. Check out this song “Rocketman” by Ironhorse Bluegrass. The HD660 S makes the banjo and guitars sound so much more lifelike than the HD650 could that I could really understand why these would be called the successor. Now, listen to the song “Sound of Silence” by Disturbed. Though this song sounds beautiful on both, the HD650 forces you to close your eyes and embrace the entire experience whereas the HD660 S shows of mainly just the singers.

But let me now talk about the individual characteristics of the sound signature so that I may be able to convey my thoughts on the HD660 S better.


From going back and forth between the original HD650 and the new HD660 S, to me, the treble hasn’t been changed much if at all. It’s still very relaxing to listen to and never gets harsh. The only difference I think exists is that the treble doesn’t balance in with the rest of the audio band like I’ve gotten so accustomed to. What I mean by this is that where the mids and upper bass are emphasized it almost over does the treble. I’ll use my oh so commonly showcased song “Diva Dance” by Jane Zhang as an example. Oh the HD650 she reaches some really high notes and the HD650 matches them. Now, on the HD660 S, though I still get the same extension per say, I just don’t get the same magic as I’m so used to. This is not a bad thing I firmly believe that, it’s just a different headphone displaying what it showcases differently (I’ll touch on that statement at the end). There’s really not a whole lot for me to say about the mids on the HD660 S. They’re as good as the HD650 but you can’t quite enjoy them as much because they’re not balanced with the mids and bass. I know my explanation of the treble probably sucked but without putting them both on your head and letting you instantly hear the “oh yeah that’s what you’re talking about” that’s the best I can do :p.


The soul of the music, where emotions are expressed and feelings are created. And that is honestly a really good analogy of what the HD660 S can do. Whereas the HD650 presents music in a very relaxed and calm manner, the HD660 S makes the artists voice front and center in the presentation you’re listening to. And it really doesn’t matter rather it’s male or female vocals you’re listening to, both sound beautiful. You can even use the Diva Dance song I listed earlier, yes I think it sounds overall better on the HD650, her voice still resonates throughout her entire performance unhindered by the HD660 S’ forwardness. Or even take the song “The River” by Blues Saraceno. The emotion of the song is easy to understand and even feel. When I just want to enjoy my music and listen to a lot of acoustic focused music as well, the HD660 S has become a go to. To talk more about instruments. Listening to the HD660 S just as its own headphone is a very enjoyable experience. The forwardness of the mids make every guitar string pluck or swipe of a bow of a cello sound so musical and enjoyable that I often get chill bumps when the solos take off.

So I really like the mids on the HD660 S and I think when considering the this headphone its own headphone, I believe most will as well.


I often call the bass the heartbeat of the music and like the mids the HD660 S plays my analogy very well. The bass isn’t overpowering whatsoever but it most certainly is more present than what was in the original HD650. The bass is heavy and full so most shouldn’t feel a need for more but it’s also very controlled and as said earlier, it’s not overpowering. It complements the mids surprisingly well and really shows off when listening to some rock music (again not a bass heavy can but it plays quite well imo).



My final thoughts on the HD660 S is that I believe that Sennheiser is making a mistake in calling it a successor to the HD650, for to me it’s not. The HD660 S is a great headphone but it’s its own personality and presents audio in its own way that I believe is way too different for it to be considered a successor. The HD800 S, to me, is the definition of what a successor is. But as an individual headphone the HD660 S is wonderful and a real treat to listen to. I do like how it can be played more friendly off mobile devices however I will say that in doing this Sennheiser has really handicapped the HD660S’ ability to scale. It still shows improvement when listening to top tier gear over a mobile phone but NOWHERE near the legendary degree that the HD650 can.

Also, make sure to check out my unboxing and review videos. They’re pretty awesome AND you getta put a face to the Army-Firedawg name. If this review helped you out at all please hit that thumbs up button for it really helps me out a lot. Till next time my friends, stay safe.


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1000+ Head-Fier
Pros: Very competent, engaging and dynamic sound. Balanced, warm and yet correctly detailed. Very satisfying.
At ease with any music genre. Historical build quality.
Cons: A bit hard on the head for me. Stiff clamping force. A bit too conservative in engineering (the frame and approach is identical to the HD580). Headstage is very proportionate but more in the head compared to HD700 or 800.
Here are my impressions about the HD660S. (I apologize for my "sketchy" English)

I love the 500$ segment: there are a lot of peculiar headphones, whith samples of flagship's qualities, usually a good musicality, with some more or less important flaws usually in the mid-high frequencies. So I learned to like listening with the Beyer T90 or the Sennheiser HD700.
The aim of the HD660S can be tricky: continue the legendary lineage of correctness and musicality of the HD6xx series, while injecting something special to justify the price tag.

A lot has already been said about construction and related things, so I will skip this with a simple consideration: it's an HD6xx headphone, period.


Let's talk about the sound: I've listened to the headphones with several amplifications, with different powers and more importantly different output impedances.


The HD660S has the sound of an headphone of the 600 series. We are far from the 700 and 800 sound landscapes, and comparison with these two headphones can basically be reported to what was written about the 600/650. Instead I will stress the differences between 660S and 600.

The HD660S has a dense, balanced, warm but still clear, contrasted, and competent sound, which will satisfy those who like a perspective close to the instruments and a more vigorous than airy sound. The headstage is more contracted and "in-head" than the HD700 or HD800, it's even more intimate than the HD600 but with a better feel of the environment compared to the latter.


The HD600 sounds airier and softer, while the 660S sounds closer and with more impact, with a more powerful bass, but also with a more prominent particular part of the high frequencies. Paul Simon's voice has a clearer tonality in the HD600, while is darker and a bit back in the mix with the HD660S. The transients are better outlined in the HD660S, which, directly compared to the HD600, appears as the more modern headphone, with better resolved frequency extremes. The good old HD600 exhibit a hint of acidity in the voices, which is absent in the HD660S. On the other hand the HD660S seems sometimes to "mute" some harmonics, while the HD600, even if more "squeaky", seems to flow more naturally across the mid frequencies. But it's difficult to say which one is closer to the "truth". The headstage seems to me more over the head with the HD600, and more intimate and "in-head" with the HD660S, which however has probably a better depth of image.

In the song Tell-Tale Heart there is a bows attack (about a minute and a half from the beginning) I often use to test the timbre vividness, while the low end of this recording is a very good test in this range. The bows rubbing "roughness" is more evident in the 600, while the 660S masks a bit of this mid-high frequency component, compensating with a more modern and less "mid-centric" sound. The bass is more evident and punchier in the 660S. Overall, in A/B comparison, the 600 sounds like the "old" headphone, while the 660S is the "remastered" one. The electric bass is softer in the 600, and has more impact on the 660S. In the orchestral parts, the 660S has finer grain and the listening point is placed more on the director's podium than among the audience. The high frequencies are refined, clear, detailed and have great resolution. Cymbals & co have definitely a very well-defined harmonic spectrum. Beautiful expressivness.

I begin to understand this new headphone. As before, compared to the HD600, the HD660S is the one that sounds with the most powerful frequency extremes, fuller and cleaner. There is a somehow attenuated mid-high frequency area compared to the 600, balanced by a rise slightly above it. This contributes to the feeling of a cleaner sound, but in direct comparison the voices seem timbrically a bit "muted". But if one get used to the 660S' sound, it seems that maybe the 660S is right, while the 600 emphasizes some harmonic components perhaps a little more than the necessary (but very pleasantly). Nevertheless, some chorus lines seem slightly better outlined with the 600, while on the other hand the 660S has better positioning of the orchestra and chorus sections. Overall the 600 seems timbrically more "phoney" sounding than the 660S.

At circa 6:30 minutes of this movement there is a beautiful effect of rubbing of the violins, which (live) gives an exceptional emotional experience, it seems to feel the freezing northern winds on the skin. It's a very difficult part to reproduce (I think it's basically impossible to reproduce the experience that you feel in the live concert hall). Given the somewhat discontinuous performance of the 660S around the mid-high frequencies, I did not expect to hear the newcomer reproducing better the breath of this magnificent moment. It is not clearer, but it's better placed in the soundstage, it has more breath, where the 600 loses room sensation (maybe for the lower bass presence). The 660S is a really good headphone.

How close is the guitar with the 660S! It's exciting but there's less sense of distance of the stage. Nevertheless, the timbre of the strings are exciting, albeit slightly resonant on the lower end, while the 600 has a more "acid" sound. The reverb of the recording venue is more evident in the 660s.


With this recording the perspective difference between the two headphones is clear. The 600 sounds more distant, while with the 660S we are virtually in the middle of the stage. It's not very realistic, but the introspection in the harmonic structure of the instruments is exciting. The 660S has the best harmonic resolution: yes, this time the 600 loses harmonic content in comparison.
But the 600 renders magnificently the voices in the indescribable Introitus of the Requiem A 15, while the 660S seems to lose some expressive nuances.

The performance of the 660S is really remarkable: it's punchier, more contrasted and extended than the HD600. (the sound of the cymbals is timbrically almost "suffocated" in the 600, while it's exceptionally harmonically resolved in the 660s, in absolute terms). The "on-stage" feeling of the 660S does a good job here. The 600 is very nice, but can't really compete with the 660S here.

I think that the new HD660S is a remarkable headphone, pleasing and enjoyable to listen to. Its dense, and contrasted sound balance it's not as "heavy" as the 650, it remains way more agile and can really be an update of the 600, except maybe for a "too near" sound perspective. The closer but better sized perspective is exciting, except when it becomes too "dense" (especially if you're used to the landscapes of the HD800). The grain seems finer than the 600, and the resolution of the high frequency is better too. Sometimes there are some timbrical and expression information that seems slightly "muted", but it is a very little price to pay compared to the overall performance. Also because sometimes it's the opposite, and it's the 660S the more expressive headphone. In terms of "music flow", the more "shy" 600 seems sometimes to perform better, but in A/B comparison it sounds more "mid-centric" and less resolved. The 660S is clearly the fuller-sounding headphone. The harmonic resolution on cymbals, triangles & co is absolutely excellent.

I expect great success for the 660S, it is perhaps one of the most balanced headphone in its price range, able to span on all music genres without fear. Its "500$ specialization" is this warm, dynamic and contrasted balance, coupled with a beautiful timbrical accuracy.

- The HD700 is less correct and less timbrically competent, but its soundstage, warmness and the particular high frequency detail gives to this headphone a somewhat special status (which you can love or hate). If you like its sound perspective, the Hd660S can't compete. Instead, if strict timbrical accuracy is what you seek, the 700 will have some difficulties to compete with the HD660S.
- The HD800S is the flagship one and has another approach. A/B compared after getting used to the 660S it seems almost too light; 660S instead seems to be too closed and dense if you get used to the sound of the 800S. Without the "ear habitude" bias, there is no match for expressiveness, elegance and rendering capabilities of at least all the acoustic situations, in favor of the 800S, not to mention the huge difference in the soundstage, which gives the necessary breath to the symphonic orchestra, something the 660S has some difficulties to deal with. Moreover, the 800S clearly dig deeper in the bass.]


Member of the Trade: Acorn Audio
Pros: Detail retrieval advantage over the stock HD650, impactful and dynamic, bass control, least veiled of the 600 series, easier to drive at half the impedance of its siblings
Cons: Still a bit veiled with uneven treble, shouty and a bit harsh at times, narrow soundstage, not a big step over the existing Sennheiser line

Special thanks to Audio Sanctuary in New Malden, London, for lending me their display units of the HD660S, HD650 and HD600.


Impedance 150 Ω

Connector Stereo jack plug, gold-plated 6.35 mm, unbalanced, 3-pin 4.4 mm, balanced, 5-pin

Frequency response 10 – 41,000 Hz (-10 dB)

Sound pressure level (SPL) 104 dB at 1V 1kHz

Ear coupling Over-ear (circumaural)

THD + N, total harmonic distortion and noise < 0,04% (1 kHz, 100 dB)

Transducer principle (headphones) Dynamic, open

Weight w/o cable Approx. 260 g (without cable)






Build Quality & Features

260 grams feels like nothing to me now after such a long time with the Focal and ZMF headphones. That being said, I find the comfort to be largely affected by the amount of clamp these can have while new - a few hours in and I was feeling the pain. Once on your head, however, they stay put so you can headbang away without fear of them falling off. The cushioning on the headband is adequate, considering its low weight.

As for the build itself, it’s very much in line with the other two 600 series headphones. Made of plastic, the HD660S won’t wow you with its durability – but take reasonable care of it and it doesn’t seem like it will fail you. Aesthetically, it’s my favourite of the three – or four if you count the Massdrop x Sennheiser HD6XX as well. The black is sleeker than the marble finish of the HD600 especially. A change on the cups is the addition of the logo, and another change (one I personally dislike) is the logo on the headband being made smaller and more to the side rather than squarely in the middle.




It comes with…the same exact cable connectors as the other two. In a way, I’m sure this is appealing to longtime fans who have spent on aftermarket cables that they’d like to carry on over – but the connectors on these are some of my least favourite available for any headphone.


(Schiit Wyrd > USB Version 5 Schiit Yggdrasil > Dragon Inspire IHA-1 > Sylvania Bad Boy 6SN7 + Philips Metal Base GZ34)

The most noticeable aspect of the HD660S, at first, is its capability of delivering an impactful sound. The attack of notes is quite well done, with the announcement of each percussive sound not shirking. The interesting thing about this impact is tied squarely to the bass of the HD660S. There isn’t a lot of bass in amount, but it seems quite focused and punchy. Most importantly, it isn’t sluggish and moves at quite a nimble pace – especially noticeable with bass guitar playing and double-kick drum patterns. I’d also characterize it as being somewhat clean sounding, it attacks fast and decays quickly without any lingering feeling or even bloom. It doesn’t extend very far down, about on level with what I’m used to with the Focal Utopia, but it sounds disciplined enough to render Angel by Massive Attack’s brooding bassline properly with surrounding instrumentation.

The lower-midrange of the HD660S sets it quite a bit apart from the HD650, to my ears – but more on that later. It is decently represented without slipping into the overly warm category. Make no mistake, I’d still classify this as a bit of a warm headphone (all things considered) but it certainly has less emphasis in the region than I expected. I found male vocals to do really quite well, and found the chugging effect of distorted electric guitars to have ample body and weight to them. Now That We’re Dead by Metallica is a good example of both these factors in play, with the bite of the rhythm guitars being in full effect.

The midrange itself isn’t lush. I wasn’t immediately hit by a feeling of sweet smoothness that I remember hearing with the Hifiman HE-500. Instead, I’d characterize the midrange as a bit shouty in nature. I’m reminded of the Focal Elear in a way, but the HD660S is leaner. The feeling of sound pushing itself out at the listener can’t be escaped, and just like with the bass – the headphone sounds impactful and dynamic. The dynamic nature of the HD660S is actually very noticeable, especially compared to its siblings, as being laid back is not in its nature. As stated before, drums hit hard – but when everything else joins in, it can lead to a fist-to-the-gut experience that would probably not appeal to fans of the pre-Fazor Audeze LCD-2. To my ears, this is the most resolving of the three Sennheiser 600 series I’m talking about today, narrowly edging out the HD600 due to its better bass reproduction, speed and dynamics.


The issue that comes to mind, however, is that the shouty nature will definitely not be to everyone’s taste – and comes across as artificial across the board. Yes, this is not a very effortless and natural headphone – but rather a brazen attempt by Sennheiser to inject a livelier sound into their 600 series. There are casualties along the way of this approach, as some more aggressive genres (depending on production and mastering) may sound downright harsh on the HD660S. That same lower-midrange texture that helps chugging rhythm guitars in the lower registers will morph into a shrill upper-register experience for anything that isn’t recorded, mixed and mastered to meet audiophile requirements. This isn’t only on the music itself, however, as even the harsher moments of The Downward Spiral by Nine Inch Nails can sound a step too far at times – and that’s a very meticulously put together album indeed. What’s interesting is that genres like classic rock and jazz definitely benefit from this added aggression, and the separation of instruments doesn’t descend into a hodgepodge very often no matter the source material. The string section of Comfortably Numb by Pink Floyd ebbs and flows properly along with the song, something I don’t hear as much of on the HD650 but definitely hear more of on the HD600. The detail retrieval upgrade is in effect too, with the Nashville-tuned acoustic guitar in the background being more noticeable on the HD660S – another indication of it being more resolving than its siblings.

That brings us to the Sennheiser Veil, a concept that I’ve seen either wholly accepted or argued over (usually by owners of the HD600 and HD650) on audio forums. I’d like to state that I do hear it with all the 600 series, and I still hear it with the HD660S – which might be bad news for anyone wondering if this new iteration “cures” that. The interesting part of the treble on the HD660S is that, while it is still slightly veiled, it puts up a hell of a fight – for better or worse. Keeping with the dynamic nature of the bass, the treble causes cymbals to hit hard and have some severe impact – while being more noticeable and less chopped than the HD650. Adding to my belief that it’s the most resolving of the three (overall), the HD660S has good attack in this region and decent decay – not many situations of cymbal bleed, to my ears. This is another reason I enjoy jazz quite a bit on this headphone, and even the jazz-metal fusion that is Black Sabbath by Black Sabbath is well served by the capabilities of the HD660S – with all the cymbal work being brought out quite properly. To me, personally, it’s a fun instrument to listen to and I often look for its presentation in audio gear.

The issues are, firstly, in line with the treble being slightly hard in nature and a bit artificial (this has become a running theme at this point in the review). Most importantly is what sounds like a slightly brittle characteristic that I can’t seem to shake off hearing. The HD660S is far from the worst offender of this in the world of headphones, but it’s noticeable enough to mention. Again, the worst pairings of this seem to be more compressed, loud and slightly abrasive music, and I’m sure someone will argue that this is more the fault of the source material than the headphone – hell, they might even call the headphone extra revealing for this. This is not entirely the case, as the ability of headphones to keep things together at the seams when faced with such a recording is important – to me anyway. Yes, headphones can be unforgiving or reflect the harshness of a recording like Holiday In Cambodia by the Dead Kennedys – but I honestly feel that the HD660S takes it one step too far in the negative zone. The song sounds worse than a demo recorded on a tape recorder in an echo-laden space when listened to on the HD660S. Even with the notoriously picky HD800 back in the day, I didn’t get nearly the same effect – which makes me think the symptoms are a combination of the shouty nature of the headphone coupled with a slight uncleanliness in the treble. This also makes me ponder why it is that a song like that can hurt to listen to, but I can immediately switch to something like Dark Side of the Moon and wish that there wasn’t a slight ceiling on the sound (the veil in effect). This selectiveness adds to my feeling that the treble extension is a bit uneven.

So how does it all come together? If I was to judge the HD660S on its own, and I do apologize for all the comparisons to its siblings above, I’d say it was a capable headphone. Even with the glaring shortcomings, I didn’t detect any massive deal breakers – for me personally. Sennheiser dragged the several aspects of the HD660S, kicking and screaming, and locked them in the chassis and told them to play nice – and for the most part they tend to. It’s a dynamic and impactful experience with a decently resolving nature that is capable of good detail retrieval – but how does it really stack up with the rest of the lineup? Sadly, I’m not fully equipped to answer this question as I really have not participated much in Sennheiser 600 series culture, beyond owning a HD600 for eight months way back before I even started reviewing. When I mention 600 series culture, I am not exaggerating – there is a large amount of people who are committed to having and sometimes modding their HD650 experiences. There is also the belief that the headphone scales with gear several times its MSRP in value, leading to the “infinite scaling” meme that we nerds in the audio community know of. Before I offer any insight that I can gather, I’d like to mention a few things:

1. I’d like to stand by my audio chain, with which I’m very familiar at this point and believe is more than adequate to compare these headphones.

2. These headphones are all stock display units borrowed from my local store. No mods are in place, and I haven’t heard any modded HD600/HD650s either. I aim to only speak to what I hear on this specific setup with these specific stock models.

3. Strong believers of burn-in should know that the HD660S is a very new unit while the HD650 and HD600 are much, much older.

4. I have heard that the age of the pads makes a difference – both the HD650 and HD600 pads are not decrepit and have been changed not too long ago by the looks of them.


(Schiit Wyrd > USB Version 5 Schiit Yggdrasil > Dragon Inspire IHA-1 > Sylvania Bad Boy 6SN7 + Philips Metal Base GZ34)



This headphone, in the form of the Massdrop x Sennheiser HD6XX, and its ability to be obtained for $200 is a huge selling point in 2017 – but it’s been a staple in the audiophile community for far, far longer.

The first thing I wanted to test was the belief that the HD660S was a “HD650 with sub-bass” that I’d seen floating around the online community. I compared the two with the 3:27 mark of Why So Serious? by Hans Zimmer off of The Dark Knightsoundtrack. This deep and rumbling sub-bass section can sound downright muted on some headphones. While it isn’t so on the HD650, I found the section to have far better body and control on the HD660S. It’s important to note that it isn’t just sub-bass rumbling that’s going on here, but a very low string section accompanying it – building to the near-nauseating suspense in the track. The HD660S was able to represent those aspects along with the bass at a more refined level than the stock HD650 – due to its superior ability to separate the bass with the accompanying frequencies. On the HD650, I felt like I was missing out on both some impact and perhaps some of the notes themselves – as the overall experience was quieter. As you may expect, the softer bass impact of the HD650 extended to everything else that I listened to – and the feeling was a bit odd coming straight from listening to the HD660S for a while.

The softer bass impact and less intricate separation on the HD650 highlights its lesser ability to separate the bass and lower midrange compared to the newer headphone. While there isn’t unfathomable bass bleed, there is a distinct feeling of melding going on that is better handled on the HD660S. This leads of the overall sound of the HD650 being quite warm, certainly compared to its two siblings. I found the midrange to be an easier listen than the HD660S, however, as it has more body to it. While it’s definitely thicker here, it lacks a bit of the texture that the HD660S has that lets distorted guitars sound as raw as they should – there is a feeling of smoothing throughout on the HD650. Also there’s a certain boominess that is present at times, Now That We’re Dead by Metallica has the bass guitar and rhythm guitar crunch meld to be slightly bloated.

I found the HD650 to be a more veiled experience than the HD660S, but it also lacks the very shouty nature of the newer headphone. It also doesn’t possess the treble harshness that the HD660S can have at times, Holiday In Cambodia is on the opposite end of the spectrum when it comes to its reproduction. Instead of harsh like on the HD660S, it’s overly smoothed and distant instead of biting. Also, both headphones have similarly intimate soundstage – but I feel that the HD660S has slightly superior imaging with there being less of a blind spot in the center.

The lessened detail retrieval is also heard on cymbal patterns. Gloria’s Step by the Bill Evans Trio on Sunday At The Village Vanguard sounds more immediate in its percussion on the HD660S. Not only that, but the cymbal impact and decay is both softer and even a bit rolled on the HD650 by comparison. The snare sound on the left channel is slightly intruded upon by the upright bass guitar work as well, reminding me that the HD660S indeed separates better.

Couple all of the above with the fact that the HD650, in stock form, is so veiled and it should be a no-brainer that the HD660S is the better headphone right? Well, it isn’t that simple. Price-to-performance and mods aside, the HD650 might be more to the liking of several folks simply due to its more cohesive overall sound. I’ve quite easily ticked off ways in this review where the HD660S does things at a more competent level than the HD650, but the overall feel of the older headphone seems more even to my ears – and I’m not a fan of the 600 series much at all. Again, this is very subjective – but I think that the various little triumphs that the HD660S has in this comparison don’t entirely add up to a decisive advantage overall. It still is the shoutier of the two, the more treble uneven of the two and the more artificial of the two. The HD650 is more laid back, and dare I say a more comfortable listen? My personal tastes, with the Utopia being my favourite headphone and all, actually fall more into the presentation of the HD660S camp – but my nitpicking nature can’t entirely overlook the treble and tonal issues in this comparison. After all, I don’t only listen to jazz and classic rock.



What I enjoyed immediately after switching to the HD600 was this sense of depth the sound gained over the HD660S. While the newer headphone is punchier and more impactful, it can sound thin and strident compared to the HD600.

The HD600 is the most midrange-centric of the three headphones, and it has my favourite presentation in this regard. The reason for this is, firstly, the lack of lower-midrange and bass bloat that I hear with the HD650. Secondly, and most importantly, the presentation is a step above the HD660S when it comes to an effortless and natural sound.

The bass of the HD600 is the most muted of the three headphones and it remains largely in its own camp and doesn’t intrude like the HD650 can at times. There is still a heft in the low end of the headphone that doesn’t completely eliminate the sensation of bass, but it’s hardly ideal for genres dependent on slam and rumble – for which the HD660S is a better choice. Overall impact is also less frantic than the HD660S, but seems slightly more pronounced than the HD650. The sub-bass rumble in Why So Serious? is also weaker and less controlled than the HD660S, but the instrumental has a certain sheen around it that is interesting. It’s not smoothed, like the HD650, but it’s definitely a softer and less harsh experience than the HD660S during the crashing moments.

This distinct lack of troublesome glare, compared to the HD660S, is not to be understated. It, coupled with the lack of bass bloat, makes this my favourite of the 600 series. The HD660S has a more open sound, and retains more texture overall – but the musicality of the HD600 is hard to beat for the newer headphone. Not only is the HD600 more cohesive and natural sounding, its detail retrieval is no slouch itself. Also, the treble is more even up to the point where it falls off the veil cliff – the HD660S extends just a bit further before meeting its own similar demise, but the road there is rockier.

Just for clarification, the soundstage of both headphones is similar – but the imaging on the HD660S seems less hard panned to me than the HD600.


Final Thoughts & Conclusion

I feel like this is a good time to briefly explain my mentality going into these reviews. I don’t do these to offer purchase advice for my readers/viewers, but rather to express my thoughts and opinions on audio gear. It’s largely curiousity based on my end and I find it difficult to answer queries asking me to decide on gear for people.

I’m stating this now because I have no idea if the HD660S is a worthy purchase at its MSRP price point, either for fans of the 600 series or for people looking to buy any of the three as an introduction into the Sennheiser line. I can’t contextualize entirely what buying the HD660S means, at its current price of $500, over the $200 HD6XX/HD650. One reason for this is my unfamiliarity with the HD650 scene, and the many mods I see mentioned online. I’ve heard that the HD660S pales in comparison to a “KISS modded” HD650, but I can’t confirm or deny that personally.

I do feel that these headphones are distinct, however, each with their strengths and weaknesses. I have a minor gripe though as I think Sennheiser’s changes might not go far enough. With all the advantages and disadvantages considered, I can’t say it’s a better headphone than the other two – but if you factor in the cost differences vs. your own preferences, you should be able to draw a conclusion as to which suits you the most.

The Sennheiser HD660S is the most impactful and dynamic of the three headphones, it has the strongest ability to expose the details in tracks and it has the most controlled and emphasized bass. However, it has issues with being shouty, it is still veiled (although less so than the HD650), it has less of a natural presentation than the other two and it is a large jump in price. A day will come when all three headphones will be within an arm’s reach of one another in price, and then you can try all three and decide accordingly.

Excellent review. Unfortunately the HD660S while having some superior traits to its predecessors as a whole is not as good as the earlier HD580/600/650, never mind the early HD580/Jubilee. The basic problem is its harsh, hard and unnatural presentation. These problems remain with higher fidelity upstream. The lower impedance hardly makes any difference either. So unless you like aggressive headphones it's much cheaper to get an older version.
Uh. "The basic problem is its [660s] harsh, hard and unnatural presentation" - I'm just gonna ignore that comment. You alright there pal? How do you even come up with this?
Really strange review, 660s as a harsh, shouty heaphone? its just a nonsense. Actually the 600 and 650 more shouty because of the more elevated higher midrange. Maybe the reviewer just mixing shoutiness with clarity or stg like that.
I can understand that clarity could be strange for someone who used to the slightly veiled sound of the older Sennheiser 6 series. But in this case its more like a reviewer issue, not headphone flaw.