Sennheiser HD 660S2

I wanted something different then my sundara, so I thought I give the HD660S2 a try to see how it sounds to me.

Inside the box ,the HD660 S2 comes with two sets of cables, the Single End cables and then the 4.4mm Balanced cables, a 6.3mm to 3.5mm adapter which is a short cable, and finally a carry pouch.

Sadly there’s no 4.4mm to XLR adapter or an XLR balanced cable in the box, So if you wanting to run them balanced with a Dac/AMP or headphone amp lacks 4.4mm but has XLR, your going to have to either buy a 4.4mm to XLR adapter or a XLR balanced cable. Unfortunately I had to my self.



The first thing I did was see how light the headphones were, and they were super light, to the point I don’t feel them on the top of my head, unlike my sundara, I found the ear-pads to be very comfortable on my head, while wearing glasses. I do wish the ear-pads them self were a little bit more deeper, but it not really a problem. Durable wise they feel well made, But I wouldn’t want to drop them on any hard surfaces or too many times. The cables quality feels good too.





I had a little bit of trouble plugging in the cables into the cups, as it as a little tight, but evenly I got them in. Scared me a little bit as I thought I was going to break something.





Source used with the HD660 S2 was the EF499 with it set to Nos filter setting.





Hows it sound

Right off the bat, I don’t find them bright at all, to me they are smooth. With really good details, None of the instruments bump into each other, they can be heard separately. I feel like the sound stage is kinda large as it sounds like to be outside my head.



Its so easy to hear where every thing is located in a song. Including other singers voices, they also don’t overlap each other and isn’t thin, but have right amount of weight behind them. Singers feel like as if there very close to my, not right up in my face but close I would say.

I found the over all bass to be very good, it doesn’t bleed into the other frequencies, I would say its punchy, its the only way I can put it into words.



Games wise it works really well in games, in dead island 2, I had no issue with telling where zombies were, rather they was behind me, on my sides or in front of me. Gunshots and other weapons had enough behind them, it didn’t sound like my character was firing a cap gun. Sounded really good in the games I been playing.



Conclusion



I find my self enjoying the HD660 S2 a lot, I don’t regret buying these HD660 S2, I find my self enjoying it more then my Sundara.

Sennheiser has a unique problem: it is its own competition in many aspects.

Let me further clarify that statement.

The HD 650 were released over two decades ago. Even now they remain relevant, a staple even when it comes to open-back dynamic driver headphones.

So, how do you follow up a bonafide classic? There is no easy answer to that question. Sennheiser has been hard at work, trying to capture lightning in a bottle one more time.

The weight of expectations bears heavier on the HD 660S2 than any other pair of mid-range headphones. Perhaps unfairly so, but that’s what makes reviewing them worthwhile.

Read on.

Shout out to Sennheiser Germany, for providing the HD 660S2 on loan for review purposes.
This review originally appeared on Headphonesty.

Packaging​

Sennheiser has simplified the packaging of the 6xx series headphones to something rather generic.

While the old packaging of the HD 650 and HD 600 used to come with a heavy storage box and padded innards, the newer revisions arrive in recyclable cardboard cutouts.

In the box​

• Sennheiser HD 660S2 headphones
• 1.8m 6.35mm terminated cable
• 1.6m 4.4mm balanced cable
• 6.35mm to 3.5mm adapter cable

The good news is that the HD 600S2 come bundled with balanced and single-ended cables, making it easy to connect them to various sources without having to buy an aftermarket cable.

Design​

The HD 660S2 have a mostly plastic build with a spring steel headband.

Despite the usual sneer directed at plastic for not having a “premium” enough feel in hand, the HD 6xx series of headphones tend to last for a long time. My HD 650, for example, have been going strong for over seven years.

The HD 660S2 use a “dual-entry” cable system where both earcups use separate cable entries. This system is far more robust than the single-entry ones and does not rely on wires running through the headband.



The yokes have adequate sidewise movement, and the earcups rotate front-to-back by about 15 degrees, so I think most people won’t have issues getting a good seal. The left earcup can be identified by touch thanks to three raised dots near the headband.

Once again, Sennheiser takes care of the little details.

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The earpads are the same ones that come with the other HD 6xx series of headphones, which means that the sound signature will become more mid-centric as the pads wear over time. Fret not, though, as replacement pads are widely available.

Comfort​

The clamp force is slightly higher than I prefer. However, the pressure is so well distributed that no hotspots are formed.

Internals​

The Sennheiser HD 660S2 use 38mm drivers with a new magnet design despite utilizing a previous-generation housing.

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Sennheiser uses the same “vented” magnetic system on the HD 660S2 as the predecessor HD 660S. On the older models (e.g., HD 650 and HD 660), the air displacement at the back of the driver was controlled via foam inserts.

This new design utilizes stainless steel mesh, resulting in much tighter tolerance and superior material longevity.



The driver is also similar to the 660S except for the voice coil, which is now thinner (and thus, the increase in impedance, I assume). The housing is essentially the same as the HD 660S as well.

Sennheiser HD 660S2 Sound​

The HD 660S2 are mostly neutral with slight emphasis near the presence region.

The sub-bass reach and extension are a noticeable improvement over the preceding models.

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Bass​

Sub-bass used to be the Achilles’ heel for the HD 600 and HD 650, and while the HD 660S2 improved noticeably, the latest version of the HD 660S2 delivers the best performance.

The HD 660S2 produce excellent bass for a pair of open-back, dynamic-driver headphones.

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Bass is tight and controlled while having excellent speed and mid-bass texture. Deep, low rumble until 30Hz with superior definition. However, those used to accentuated sub-bass will not find it here.

Given that the HD 660S2 are meant to be neutral, reference-tuned headphones, the sub-bass not having a +10dB shelf makes absolute sense. The slam factor is an area that can be improved, as bass notes do not have a lot of physicality to them.

Midrange​

The mids are the bread and butter of the HD 6xx series of headphones. The HD 660S2 are no exception.

Sennheiser goes a slightly different route this time, with the upper-mids being less pronounced than the predecessors.

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This results in a more laid-back presentation of female vocals compared to the hyper-forward HD 600, while male vocals are not as lush-sounding as the HD 650.

If you are sensitive to shout, the HD 660S2 will be perfect for you.

Treble​

The aforementioned lower-treble peak around 6kHz can be tiring for treble-sensitive people. I experience fatigue when listening to poorly mastered tracks. Certain rock and metal songs, e.g., Red Hot Chilli Pepper’s Californication, are susceptible here.

This peak aids in finding mastering issues, however, especially for de-essing. So it’s there for a reason.

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Fortunately, the upper-treble is well-extended and has excellent tuning. Cymbals, hi-hats, and triangles resonate naturally, with precise attack and decay. Treble timbre is great as well, with no metallic “zing’ or “sheen” affecting the high notes.

Overall detail and resolution are very good, but the HD 660S2 fall behind some mid-range planar magnetic headphones.

Soundstage and imaging​

Soundstage is average for an open-back, but these don’t sound as closed-in as the HD 600 or HD 650. Stage depth is also improved, with frontal projection better reproduced.

Imaging is precise with accurate instrument placement.

Dynamics and speed​

Macrodynamic punch is fairly good, even though these aren’t the last word for overall slam and punch.

Microdynamics (gradual shifts in volume) are excellent, on the other hand. Separation is also above average, but some mid-range planars (e.g., the Hifiman Ananda Nano) are better.

Comparisons​

Vs Sennheiser HD 650​

Sennheiser’s HD 650 are legendary in their own right and are massively popular even 20 years after launch. The HD 660S2 manage to noticeably improve the bass response over the older model.

There really is no contest – the HD 660S2 bass is in another league compared to the HD 650.

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The HD 650 have noticeable distortion in the sub-bass region, and in busy tracks, bass notes tend to smear into each other. After some modding, I have slightly improved the distortion characteristics of my own HD 650, but even then, you reach the limits fairly quickly.



Another aspect that has seen a jump is the stage width. While the HD 660S2 are not particularly wide-sounding, they fare much better than the HD 650. A reason for this is the less forward upper-midrange, while the bass extension also helps create a sense of depth.

The upper-treble is a matter of taste. I find them both well-tuned in this regard, with the HD 660S2 perhaps being a tad more resolving. However, the lower-treble is where I much prefer the HD 650.

For male vocals, the lushness of the HD 650 remain unmatched for me. For vocal-centric tracks, I gravitate towards the HD 650 for this reason, while for instrumentals and more energetic tracks, the HD 660S2 became the natural pick.

Overall, the HD 660S2 are better suited for most modern genres. At the same time, I cannot overlook the price tag, which is nearly twice that of the HD 6XX Drop variant of the HD 650. The performance upgrade hardly justifies such a massive price gap, so the HD 650/6XX still remain as good an option as ever.

Conclusion​

Sennheiser has made a practically perfect pair of studio monitors with the HD 660S2. This is what the HD 560S tried to be, and the HD 660S were touted as being, but neither could overcome the final hurdles.

Unfortunately, the price is higher than I consider “apt.” The MSRP is exaggerated for a product that is a slight upgrade over the last model – primarily offering improvement in the bass for existing HD 600 and HD 650 owners.

This puts me in a somewhat awkward position.

I really like the HD 660S2 and could live with them as my only pair of headphones – for the most part. They do not require a lot of amping (though I’d recommend picking a source with >5 Vrms and >10 ohm output impedance for best results). They sound great with every genre.

However, the market has grown so competitive that the HD 660S2 do not offer everything they should at that price. For one, I expect better staging, and the bass slam could be improved. If you find the HD 660S2 on sale for a discount, they are well worth the purchase. At the MSRP, I have my reservations thanks to the old classics.

Prelude​

Note:
Please do not read this review if you’re short on time, don’t skimp.
The HD660S2 is not a one trick pony.
Save this review in your bookmarks and read it on the train or in bed at 2am.

Audio gear has been an addiction of mine since I was 14, and I remember undergoing 4 months of research to find a headphone that will beat the Dr. Dre Beats Studios for half the price. Many days of going back and forth between many choices I landed on the ATH-A900x and to this day it still sits on my rack. I soon fell into a rabbit hole of headphones and finally considered the HD600 as my endgame for the past 6 years as I creeped into the age of 28. At this point my listening preferences have solidified and I’m no longer flexible with new sounding equipment. I’ve purchased other brands such as Hifimans to Beyerdynamics with an open mind but they have all been returned.

I’ve been satisfied with my inventory, until well, Sennheiser sent me an HD660S2. I am still unsure of what I’ve done to deserve them, but I’m grateful for having been given a pair. Do note that the following review will be as critical as if I were to purchase the headphone myself. This is the only time Sennheiser has ever sent me anything and I do not expect them to send me anything else, with that in mind I will be following up with an honest review to the best of my abilities as I would for any other headphone.

The reason I’m so picky with gear is that I’ve come to realize after many headphones, the HD600 is the absolute one especially when it comes to mids. The headphones are able to recreate the intricacies of vocal qualities. Such as chesty to breathy vocals, the ability to present vocalists’ tongue and teeth placement, and I use all that information to create a mental rendition as if I were that vocalist in front of a microphone. I love hearing piano keys actuating hammers that strike the strings of a grand piano, creating a ground for a violin to cry and soar into the highs of triangles and cabasas into the empty void of white noise. The Sennheiser HD600 has done this for me for half a decade now, and the HD660S2 is challenging for the oval crown.

On another note, my analysis on the HD660S2’s may seem overexaggerated compared to the average listener. I’ve been told before that X headphone sounds fine, or Y headphone sounds fine, or X and Y headphones sound similar. Which is fine, I’m not one to judge another's perceptions and perspectives to differentiate between gear, but please again do note that my review of course be taken with a grain of salt.

I also play a lot of games semi-competitively. CSGO/Squad/Insurgency, and I listen to Jpop(rock), Kpop, Pop, classical and almost anything involving instrumentals. Majority of my music cannot be found on Tidal, as my listening preferences are found on the asian market.

I see headphones as tools. I put on the HD600’s for music, and the HD660’s for gaming. I don’t switch to X headphone for X song or Y headphone for Y song. That has changed though. Nowadays I use the HD660S2’s for almost everything, as I now have this temptation to bring the HD660S2’s to my bedroom computer setup and the HD660S2’s to my living room setup. Every other headphone seems to have taken a step back and they don’t get my attention as much as the 660S2’s. Before we get to the gritty details, I’m going to start off the review from the very beginning of receiving the HD660S2’s.

It’s day 1.​

The headphones are still in the box, and a rather small one, and they’re in the exact same packaging as the HD560S. Well that was an unpleasant surprise. I’m going to miss the hardbox of the original 660S’s or HD600’s. It’s a shame really, I was quite disappointed by the packaging the S2’s were delivered in. Well, Sennheiser better deliver on sound, because taking out the headphone I’m also greeted by the exact same appearance as my HD660S, save for the bronze accents of the HD660S2. Financially though at least in the long run the S2’s will last at least a decade, as it will be easier to swap in all the other parts from my other HD6* headphones. The official earpads themselves are also available on amazon which is fantastic. As based on hours of swapping earpads with former Sennheiser headphones, no other earpads come close to the original earpads that maintain the original sound I have come to enjoy from their headphones.

Regarding the initial clamp, it’s quite aggressive on S2’s, and this actually hinders my ability to listen to music like my prior Sennheiser headphones where I only focus on the sound. Though the S2’s may clamp, I actually don’t notice any hinge creaking that the HD660S’s had which is great, so that’s a definite improvement. Although I will come back in 2 weeks where the clamp is less aggressive on my ears. I instead decided to opt for a sound source where I could get immersed and focused on something else.

Starting with gaming.​

Opening up discord and immediately being greeted by a few dudes in a VOIP channel, the first thing I noticed was the HD660S2 being absolutely clear for speech. This is great, because in this game I play on a daily basis called Squad, I always take up the role of a Squad Leader. The Squad Leader is essentially a volunteer managerial position where I need to listen to 2-4 voices through my right ear, speak with fellow squad leaders with my left ear, and have a 3rd party application like Discord taking up my center channel.

So was I able to differentiate between voices better than the HD660S or HD600? Yes. The only issue remaining was dealing with information overload. My brain trying to comprehend everyone at the same time and reply to each person in the correct order was a task in itself. Needless to say, whenever I’m in the zone I can reply back to every single individual with ease. Much easier than any prior headphone I’ve owned before.

Speech separation is great on the HD660S2.​

The HD660S2 also features more accurate imaging than the original HD660S. The HD660S’ imaging was already spectacular and laser sharp. I never struggled with pinpointing the location of gunfire based on audio positioning. But now it’s even better. The HD660S2 solidified the center image, and filled in the soundstage with multiple layers for center imaging. I no longer require turning my head of my game character to confirm if the gunshot came from in front of me or behind me. A short preview on tonality for later, the HD660S2’s treble is absolutely realistic, the bass is well textured and mids clear. Add this into the amazing imaging and everything sounds clean. I can look at a vehicle with its rumbling engine, or if a vehicle is on fire I can practically feel the .ogg file just radiating off it.

Sound presentation is great on the HD660S2.​

Forwarding the time to two weeks of ownership with the HD660S2’s, the clamp has finally loosened. If they are anything like my other Sennheisers, it will take about another month for them to adjust to the size of my head. So regarding comfort it will be a total of two months for them to relax and feel better to wear. I do want to point out however that Sennheiser’s oval cups and headphone design itself are the absolutely most comfortable out of all headphones I’ve tried. They are lightweight, they conform to my ears, the swivel cups are spectacular, and the earpads are just the right density.

Let me rephrase my statement from before, since the earpads and headband pads are the same as any other Sennheiser 6 line, these parts are absolutely modular and I have no issues finding them anywhere else on the market. So I don’t doubt for a second that long term ownership is without a doubt in my mind with the S2’s

The other thing I’ve noticed is that the HD660S2’s have a shorter cable than the HD660S’s, and I’ve grown fond of bringing the S2’s with me everywhere I go now. The HD660S’ longer cable 3m cable was great for walking around, but the HD660S2’ 1.8m cable I felt was a lot more manageable to have instead.

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Daily Comfort and Build Quality is spectacular on the HD660S2.​

So with that we’ll get into music.​

The HD660S2’s have gotten past the grace period and my brain has fully acclimated with S2’s, they are past the new toy syndrome. So let’s get into what it’s like to get into the HD660S2’s after never having experience with higher end headphones. I’ll be honest here, and at the time of writing, HD660S2’s are a tier up from the entire HD6** lineup and it all comes down to microdetails. Anything off to the side of the far edges of the given soundstage is solid and I have zero issues tracking those instruments.

With all other headphones it seems as though instruments would fade away into the distance. Instruments with the S2’s would exist in its own space and direct all their sound towards me in all of its clarity. Instruments in general have greater detail than my other Sennheisers and it gives them that extra bit of realism. Any reverb near the end of a song can be heard, and artists that sneak in a small vocal line are audible. I don’t need to increase my volume to hear it; no detail escapes the HD660S2’s. I’ve never heard detail like this before, and it just blows my mind listening to songs that I had in my library for over 5 years.

HD660S2’s microdetailing and intimate soundstaging is mind blowing.​

Let’s get into tonality starting with treble. The HD660S2’s bring out a smooth and realistic presentation. I can’t say for sure but they could be on the same volume as the Sundaras regarding treble, but they peak in the absolute perfect place in comparison. Sundara’s treble is somewhat grating for me and peaks in the weirdest places that seem to alter speech. Listening to that guy on AfterPrisonShow with the Sundaras is a real problem, I can’t stand listening to him with the Sundaras as the lower treble peak on the headphones emphasize the way he pronounces “S” sounds through his molars. I’ve sold the Sundaras a long time ago and my goodness just typing about that headphone brings me grief again. The 660S2’s treble regarding speech on S pronunciations are smooth in comparison, there is no grain and sibilance.

​

I digress, let’s go back to the 660S2’s treble. Got off topic there.​

The S2 has smooth and delicious treble for high hats and higher pitched instruments. It's done completely different compared to the 600, 660S and 560S. And it's realism. There was an odd familiarity to high hats with the 660S2. High hats in real life aren’t all smooth and sparkly like the HD600, they’re not sucked out and sparkly like the 660S, they’re not splashy and grainy like the HD560S.

The S2’s treble is clean, well defined and solid, they produce air when two cymbals separate. So when a drummer separates two cymbals and taps the top of a cymbal the first thing I hear is the tap, the next thing I hear is the top cymbal dancing on the edges of the lower cymbal and it just constantly creates air. It’s aggressive, and a beautiful evenness between the top and lower treble is where the treble on the HD660S2 takes the cake. This is not to be confused with the HD600’s higher placed treble on the frequency spectrum, the HD660S2 takes a middle approach. Treble extension is absolutely there but not as pronounced.

Well how does the treble on the HD660S2’s perform with the highest of frequencies? If anyone is familiar with synthesizers with jpop and kpop; the artists do not let off on these. They will sweep synths from 7khz all the way up to 14khz and sweep these tones from the left to right channel. Tracking these made me smile as the S2’s are absolutely quick and I can tell if the synthesizers are square or sine waves. Just for giggles I ran a frequency sweep from 20hz to 20khz and apparently I can still hear up to 19khz and feel 21khz. Cool! I used to be able to feel 22khz on an AKG Q701 back when I was 18 but you know what, I’ll take it, good to know my hearing isn’t completely smashed, lol.

The treble on the HD660S2 is absolutely fantastic.​

Admittedly I am not a bass-head, maybe sometimes. I don’t desire oodles amount of it as I am a stickler for bass-bleed. If anything touches my mids you will for damn sure be notified of it. I will inform you of that in a moment but bass comes first.

The HD660S2 has extension and punch. I’ve done a bit of Oratory EQ on the HD660S and I would say the Oratory HD660S is on par with the HD660S2 strictly regarding sub-bass extension. For detail they’re about the same for the Oratory HD660S too. Remove this EQ though from the HD660S and they’re back to sounding rolled off by quite a bit. The HD660S2 extends, punches, and is naturally filled with bass detail without needing EQ. Compared to the HD560S, the HD660S2 has more bass presence and detail by a long shot, it’s not even a competition. The 560S also somewhat lacks that punch as well that the HD660S has. When listening to a heartbeat on an HD660S2, I can hear the flow of blood, ventricle valves opening and closing, the resonant heart thump is all laid out for me. The other Sennheiser headphones don’t really portray this sort of detailed emphasis.

Sundaras are definitely left in the dust, if they couldn’t touch the bass weight of the 660S’s, then they definitely can’t touch the 660S2’s.

Bass is great on the HD660S2.​

If you have been skimming past the wall of text.​

You need to read this word for word when I speak of the mids for the 660S2’s. Senny fans who are all about the mids, if you came from the HD600’s, HD660’s or HD560’s; read closely.

• The HD560’s mids are linear and clear. They just get cut off by their sibilant and grainy treble that occasionally hinders them from a totally smooth vocal presentation.
• The HD600’s mids are warm, smooth, engaging and airy. Yes the upper mids can be slightly aggressive but if you pair the HD600’s upper mids peak with the right solo vocalist, you will be floating in an intimate void of smoothness.
• The HD660S mids are warm, flat, intimate and aggressive with plenty of detail and spectacular vocal separation. Not clear like the HD600’s and HD560S’ though, but if you want to have fun with multiple vocalists the HD660S is the one as the smoothed out and even upper mids allows you to pick and choose the vocalist you would like to listen to.
• The HD660S2’s mids are clear from the middle of the mids, like 300hz-ish, all the way up to the 3.5khz area and are linear like the HD560S. They also feature that technical ability of the HD660 that allow you to selectively listen to individual vocalists.

This is where it stops though.​

The 660S2 has lower mids that are slightly thicks on some tracks, and the upper mid extension is somewhat shorter in airy extension past the upper mids peak. The peak of the upper mids are solid, whereas with the HD600, this upper mid peak is more on the airy side. So no, you won’t be getting the full midrange experience of the HD600 with your solo vocalists.

BUT, HOLD ON. ​

Remember this is the Sennheiser HD660S2, and I said the center imaging on the 660S2 has been improved with more layers over the HD660S. If solo vocalists weren’t your thing, then mayhaps I interest you in tracking harmony vocals? On a track where the harmony vocals are practically cheek to cheek with the main vocalist, you will be able to hear each of them clearly. This is where the lower mids of the HD660S2’s come in, because the clear and lower and upper mids create such a stark contrast for higher and lower octaves that it plays well for vocals. To put the vocals of the 660S2’s in a nutshell. The 660S2’s give up some of the HD600’s vocal air and smoothness, for clarity and technical ability with instrumentals. Although I personally prefer vocal tonality at the forefront of my gear before all else, which is why I’m so picky regarding midrange performance.

I am still conflicted that my cute japanese vocalists don’t get the high upper mid extension though, but it’s not as bad as I’ve described. The HD660S2 has this thing of making everything else sound so wonderful that I find it okay that vocal tonality took a slight step back.

Vocals are an interesting rollercoaster with the HD660S2’s.​

So let’s sum it all up.

• HD660S2’s are the absolute best for instrumentals and present them in an intimate fashion.
• The Treble is absolutely fantastic.
• Bass extends, kicks, full-sounding, best bass in a Sennheiser I’ve heard so far.
• Mids, in every step of the way they are a step above in technical performance
• Tried and true design that the Sennheisers are known for since forever.
• Gaming headphones, forget the competition, this is it. 10 hour gaming sessions no problem.

I’m now almost a month in with the HD660S2’s, my least used cans will stay on the rack, and the headphones I rarely touch and don’t regret letting go will be sold at a garage sale.

In this case:

• The HD560S are a sell.
• The HD660S are a sell.
• The HD600 is a complementary keep for the spectacular HD660S2.

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Extra Stuff.​

Source: JDS Atom Stack in 1x gain, averaging 12 o’clock volume dials.
Look here man, the stack has done me good for 5+ years.

And again, thank you Sennheiser for the 660S2, best gift I’ve received this year outside of my cat bringing me a dead guppy from my fish tank, both were unexpected.

If you have questions about the HD660S2's, DON'T ASK.
I will proceed to crawl back into my cave and enjoy my music and games.

Thank you everyone for reading and for sticking with me on my lengthy review of the Sennheiser HD660S2.

See you all in a decade,
Smugsie