Sennheiser HD 660S2

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.

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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

"Girl From the North Country"

INTRODUCTION

Who am I? I was deep into Headphones around 2016 to 2019 and then went super deep into IEMs for a few years. For 8 months now I have been back going very deep into HPs. I love listening to music, I love analyzing sound and I love writing. I do reviews for me first due the creative intellectual masturbatory pleasure I get out of the process, and I want to get what is hopefully beneficial information out to others.

I do not have a website and I do reviews that I want to do. Sometimes I review items I purchased and sometimes not. I am a huge ZMF, Noble Audio, DCA, Holo Audio and Sennheiser fan as you can tell by my gear in my signature.

Sennheiser contacted me out of the blue last month and asked if I would like to be part of introducing the HD 660S2 to the community. Well, I am a huge HD600 fanboy so of course I would like to be part of introducing it unless is sucks. Sometimes I say no out of lack of interest or lack of time. Other times I get a product and send it back or sell it if I purchased it rather than doing a review. I am only interested in reviewing what I enjoy and what I think others should know about.

4 of my 5 last reviews involved a different style I have been working on. I use a fictitious interviewer specific to the company of the product I am reviewing. Vison Ears has Bob the sexually repressed and in recovery interviewer. Vision Ears has the grumpy and rough around the edge’s interviewer name Leroy. I am working on an interviewer for ZMFheadphones for the introduction of the Atrium Closed Back in a month or two. For Sennheiser, you will meet Siegfried the overly optimistic interviewer. This interviewer is based off a real person I lived with in Germany when I spent 4 years there. The goal for me is to enjoy the process. My goal for the reader is hopefully they gain a lot of knowledge about how the HD 660S2 sounds and how it sounds in comparison to the other HD 6 series headphones.



PACKAGING

Siegfried: Hey MM How about the cute little package? It is so exciting to get my sharp Hermes box opener out and with precision open a box of love! I see you have not changed since we were roommates am Kreuzburg in Bonn. Sorry about my buddy changing your oil on your Fiat Uno and forgetting to put the oil pan plug back in. You made it about 50 meters from the Capella before there was a divine like noise from the block. Those crack well.

SLC: Glad you have your sharp opener Siggie! Yes, those were the days. I thought it was a gunshot into my engine that seized up my white Uno. Nothing like having it towed to Ankum so Helmut could put a rebuilt engine in it. Enough of that package, now to the HD 660S2 package. I really like the simplicity of the S2 package. A small box is easier to store and easier to ship. Super ergonomic. I usually just take the IEM or Heaphone out of the box and touch nothing else in the box. I then store the box in a safe place, dry dark place. The opposite of where your mind is optimistic Sieggie. The soft pouch is totally fine. Keeps the dust off them. I love that a cable with 6.3 termination is still included with an adapter to 3.5. I love that a balanced 4.4mm cable is included. 4.4mm is taking over and is a must in my opinion. All that is missing is a 4.4mm female to XLR 4 pin male adapter. A lot of HP amps use XLR still. Then all bases are covered. And for me 1.8 meters is perfect I am still able to reach my HP amp and it the cable is not too long to use with a DAP. Simple and ergonomic is better.

I am a fan of keeping all 6XX series HPs with the same frame and cups. Other companies like ZMF and DCA and Audeze for example do a lot of sharing of chassis and cups. The lightness of the 6XX series HPs is wonderful. The clamp can be tight for some, but it does loosen up over time or can be loosened through bending of the strap.

I like how red is not used on the connector to designate Right. More discrete to have all black on the connectors. That brings me to those three dots! No one likes to be wasting time in the dark missing out on a special moment. The three dots on the left gimbal allow me in the dark or light to quickly know which cup to put on which side of my head.

Great simple package. Just like you Siggie! I mean that in a pure way Siggie! There will be no corrupting you! I tried after the wall came down, but the freedom never changed you! You stayed pure through Die Wende Zeit and beyond!

Side note: The HD 660S2 is for sure worth the $600 price and in my opinion the quality of sound coming from the S2 is worth more than the $600 price! I do not say that lightly!





COMPARISON TO HD 600

Siegfried: Remember how much fun we had am Kreuzberg in Bonn, looking out the window and hearing the bird’s making music in the morning. It really helped you get over those 0,5s Markie Mark!! I know you want to talk about the S2 specifically, but can you please start by comparing it to what is close to your heart; the HD 600. We would just sit and look at that marble sheen and those curves while the birds sang!

SLC: I can do that Sig. I remember the 0,5 Kölsch but not the birds. I do remember you rolling up to Kreuzberg in your Trabant! Oh, the smells that came out of that exhaust.

For my comparisons I will use the same 4.4mm stock Sennheiser cable. I have 2 of them which makes is slicker than the pain on your 1990 Trabbi!

600 is a classic. Crisp and clean and detailed with a natural tonality. My standard of what is my personal sound preference. Anyone say ZMF Auteur or DCA Stealth. HD 600 gets super close to those neutral, clarity boys. Huge words IMO.

The S2 has a deeper, richer, more textured bass compared to the 600. The treble is more forward with S2. Smooth forward rather than sparkly. The female vocals of S2 are a little richer and more forward. Everything seems more forward with the S2. But, upon a longer listen it is not that the S2 is more forward. There is a sheen of glass taken away compared to the 600. There is for sure an increase in clarity and dynamics. This adds a richness to the sound.

A richness to the sound that is in the same family of what I hear with 4-5K planars such at DCA Expanse. 600 is closer to the DCA Stealth and the S2 is closer to the DCA Expanse sound signature. These are 4K HPs I am talking about. A $300 and $600 HP should not even be mentioned in the same breath, but I am, and I can because it is valid that the S2 gets close enough to the 4K HPs to put them in the same breath.

There is more bass slam on the S2. Not basshead slam but textured mid and sub presence vs the 600

Male voices are closer on S2. The tonality/naturalness of male voices on 600 and S2 are both excellent. I like the timbre on both. The S2 notes are a tiny bit more rounded than the 600. I would attribute this to the wider stage of the S2.

The timbre and tonality are extremely similar on both. I like that because I like the 600 timbre. With 600 I am thinking “wow how did they make such good sound at this price range.” With S2 I am thinking “this is so good, I am enjoying this a lot, I am not feeling I am missing anything.” With S2 I am not thinking about grabbing a 4K HP to get to the next level. I am enjoying S2 for what it is. An enjoyable engaging HP to listen to. I am not feeling any FOMO when they are on my head. That to me is incredible. Like I have said with S2 a pain of glass has been taken away vs. 600. The landscape is the same as with the 600 (tonality and timbre) but I see the landscape in a clearer and more expansive way.

On the note, with staging, I am hearing the music more to the L and R on the S2 vs. the 600. For the 600 the music is right in front of me a bit at the level of my forehead. For the S2 the music expands out to the sides of my head and a few inches in front of me and higher up. At the top of my head level. The music is surrounding me more than the pinpoint positioning of the 600.

When I go back and forth for longer sessions and short back and forth, I find that the 600 and S2 have the same level of “when am I going to get tired of this” happening. Neither is warm in the “I could just listen forever and even doze off” vein. The S2 and 600 both have me enjoying the music and admiring the notes. The S2 does it with a more around me sound. Wider vs. the narrower sound of the 600. The S2 has more levels of dynamics happening. Both are micro masters. Both are tonality masters. 600 is on a narrower stage with lower ceiling. S2 is on a wider stage with higher ceiling and better 3D sound. The notes are in a bigger space vs. 600. 600 is more intimate and better for fun under the covers. S2 has you using all the bed for your enjoyment. You still with me mein Schatzi Sigi!



COMPARISON TO HD650

Siegfried: I am with you Schatz! You always preferred the HD600 in the past to the HD650. I prefer the HD650 for warmth and longer sessions and that mid bass bump. And by the way my Trabi finally no longer rolls down the cobblestone streets. It met its demise in front of the Sparkasse we lived above! What a joy life is and what a joy my Pia and Trabi have been. I donated it to our local village next to the Rathaus where we had der Richtkranz when it was being built. We sang traditional songs all night and enjoyed our 0.2 drinks!

SLC: I assume you are talking about the Trabi and not your wife Pia being donated! Congratulations on the change either way.

The HD650! I have owned it a few times. I get the “classicness” of it also. I get how some prefer the HD650 over HD600. I get how the HD650 has that chocolate syrupiness of the notes. It has a milk chocolate decay of the notes vs. the dark chocolate decay of the 600. I could write a book on the differences of these two classic HPS and put it in the museum next to Pia. I mean your Trabi. Sorry about the Freudian slip Sig man!

I could listen to the HD 650 for hours and not get any fatigue. With the 650 you must come to the notes . In comparison to the S2, with the S2 the notes come to you. Not in a an annoying “why is this guy always wanting to talk to me” at the party feeling but in a “I am so glad she enjoys coming over and engaging in a conversation with me” feeling. The S2 is closer to me. More intimate than the 650. That is wonderful unless there is bad breath involved. Nothing personal Siegried. We all get that at times.

There is a difference in timbre from 650 and S2. S2 is closer to the 600 tonality/timber. 650 has milk chocolate thicker notes. More pouring of honey in the notes. A little more distance chill I just need to relax going on. S2 notes are tighter and faster. The vocals of the S2 are closer to you than with 650.

The big similarity is in the bass. The S2 does have a milder subbass bump of the 600 but has the mid bass jump of the 650. Someone took their time to get this S2 right. They lifted some of the “boringness” of the 600 but kept the tonality and clarity and detail of the 600. They also added to the S2 the midbass bump. Great touch.

The S2 is quicker with the notes vs. the 650. The S2 has everything coming at you closer than with 650. So much more dynamic and with a pane of glass taken away. It is weird but the S2 is like an improvement to the strengths of the 650 as well as an improvement of the strengths of the 600. If you do want the music to just be there and be playing and be chill about it then the 650 is still a good choice as a complementary HP to have in your stable.

The HD 650 is more laid back with a longer decay. The S2 is so much quicker. Like going from a planar to DD HP. The tonality of both is different but “realistic”. Just slower on the 650. The S2 has a higher fun factor over the 650 and 600. I hear the heritage of the two in the S2. The game has just been upped this time. The 650 and 600 are still classics and still should have a place in an HP stable. The S2 is an updated version of both in a good way. As I said before, the developers put effort into making an upgrade and not a side grade and they really were cognizant of not ruining a classic.

Let me reiterate, that I have no skin in the game. I do this for the cognitive brain orgasm I get out of analyzing what I enjoy. This is a joy to write, and I hope informative for the listener.

Oh yes back to Sig the man. I almost forgot about my overly optimistic bud. I got deep into my audio world. I remember the time I tried to turn you into Sig the Swig and took you to the De Wallen in Amsterdam and then the Reeperbahn in Hamburg. I love how you turned the events into conversations and nothing else. You are about dignity and respect Herr Schlosser, and you will go to a better place if there is one, when this is all said and done. You taught me a lot.

BTW here the playlist I use for understanding HPs for review purposes. These are the songs I listen to most due to the quality of the recording in a lot of cases as well as how much I enjoy the songs:

https://tidal.com/browse/playlist/c3bde940-1a74-4096-a3f6-89ea47ac5fa1

A big take away for me in this comparison is the amount of lineage the S2 has from the 650.



COMPARISON TO HD 660S

Siegfried: Getting access to your playlist reminds me so much of the cassette playlists you would make for me. I did not know what the words meant but I enjoyed the music. I remember you asking me to stop singing “Walk on the Wild Side” in public. Seeing you blush was worth whatever I was saying. On that note, MM it is time to compare the 660S2 to the 660S.

SLC: Hey Sig check out the movie “Hedwig and the Angry Inch”. Leave your optimism adjacent to the door. Speaking of Lou Reed, I remember when I was living in France with my partner at the time and her younger sister singing the Vanessa Paradis version of “Walk on the Wild Side” and her having no clue what she was saying. That was not as funny at all compared to your version.

S2 and S time it is: Easy to cut to the chase on this one. I am truly amazed how the S2 is a better version of the S with some serious lineage going on with 600 timbre and 650 rounder notes and bass bump. All 6 series HPs have mids that are sublime. IMO most of the notes I want to hear are in the mid range. That is where the maturity of the music is at. The 6 series keeps the mids as the meat. The S2 bass is very similar to the S bass except the S2 bass has more texture. It is simply a better version of the same bass. I am not the best at talking about treble and I hear a very similar treble between S2 and S. I am not hearing any harshness with S2 nor S. But I have not met a treble I did not like. Especially after a drink or two.

As I compare the two quickly and after longer sessions with both, the big take away, which I alluded to already is timbre. The 660S2 is like watching a good movie. You are absorbed in the movie and enjoy the movie. You become one with the movie. When a movie is missing some things, I start thinking about the acting or the plot or just start thinking about something not related to the movie. I get that “something is missing” feeling. With certain HPs I become one with the HP. With the S2 I am enjoying the music. Not a laid-back music that gets boring for me. Not thrown in my face notes that get annoying. I enjoy the music with the S2. I am not thinking about the “acting” nor the “plot”. I am enjoying the music.

I do not get the same level of engagement with S as I do with S2. S is very good, S2 is so much more engaging though. The timbre and how the notes end are what makes it more engaging for me. The acting of the notes is at a better level. I am enjoying the acting and the story rather than thinking about the acting.

Separation of notes are better with S2. The moving of my head is happening with S2. In a dorky way at 5:00 am in the living room. Again, I am sounding like a broken record, but someone really took the effort to make the S2 better than S in every way and keep the lineage/strengths of 600 and 650. There is a place for 600 and 650 to continue being made. I see no reason to make the S anymore. I get that Bangle BMW feeling when I listen to the 660S. With the S2 I get the “they are back on track and a lot more” feeling. The S2 tops the S in every way and leaves no need at all to grab the S for a session. S2 is simply that good.

“They say, oh my God I see the way you shine
Take your hand, my dear, and place them both in mine
You know you stopped me dead while I was passing by
And now I beg to see you dance just one more time”
Dance Monkey by Tones and I




SENNHEISER HD 660S2: Final thoughts

Siegfried: Time to say adieu MM. I thank you for helping me see the dark light but keep my optimism. I have enjoyed my time with you again. It has been a while. I ask you to wrap up your thoughts about the Sennheiser HD 660S2. I now look you in the eye and say Prost and Zum Wohl mein Freund!

SLC: Sig, you are the man and cheers to you, my friend!

I listen to jazz more than anything else. Trio and modal jazz to be more specific. My playlist for understanding gear does not have a lot of jazz in it since I want to hear numerous instruments happening in the same song and more bass and treble and of course vocals.

The HD 600 is an amazing HP for Trio and modal jazz. Slower lyrical jazz with fewer instruments. The detail and timbre are superb for that purpose. The HD 650 draws me to Rock and Roll and the sick amount of Americana music I listen to (in a simplistic way Americana caught on with Johnny Cash and Bob Dylan with “Nashville Skyline” to Lucinda Williams and of course the demigod Neil Young. The music and producing by Dave Rawlings are a great current version of Americana).

The 660S2 is drawing me to everything. In a similar way the ZMF Caldera does. Nothing is missing in my opinion. Nothing is too much. The timbre is there. The musicality is there. It is detailed. The bass is there. There is not an annoying V shape. 80% of the music is in the mid-range and the mid-range of the 660S2 is exactly what it should be. The bass and treble act as accoutrements. The top and bottom frequencies are super well integrated with the mids. They add color to the mids. Not too much and not too little.

I am not one for hyperbole when it comes to audio gear. There is a lot of it out there and a lot of really good gear. Most gear has strengths and weaknesses. I am hard pressed to find a weakness with the HD 660S2. Maybe because it is harder to drive. But it sounds great on all gear I have tried it on from a normal DAP to high-end Headphone amps. I think they went back to 300 Ohms because at 300 ohms the magic of the driver can happen. I think Sennheiser tried to keep the magic by appeasing some and going to 150 Ohms with the 660S. I think the 660S is a good HP but the Sennheiser 6 series magic was gone. I can say with 100% SLC MM certainty that the magic is back.

The magic is back with a style of its own. The 600 lineage is in the 660S2. With even more 650 lineage in the 660S2. That said it does not replace the 600 and it does not replace the 650. They both have huge merits and should be kept in the lineup. I reach for the 600 when I want to focus on detailed timbre and sub bass. I reach for the 650 when I want added mid bass and warmth and musicality. The biggest thing the 660S2 adds to the mix is added dynamics. Sound is happening in a 3-dimensional way that other HPs in the 6 series do not have. All around you but with wonderful timbre/tonality and musicality. Also, with detail and warmth. Sometimes the engagement factor can be lost when you add all that together and this is not happening with the 660S2. That is what makes the HD 660S2 so special.

Time for me to go listen to Dance Monkey followed by the Dylan/Cash version of Girl from the North Country.

Until we meet again!
MM

The review sample was loaned by the local Sennheiser distributor for writing a review for "hxosplus" printed edition.
There was no obligation for an international review but I have decided to share my impressions with the community.
As always I haven't received monetary or any other kind of compensation and I don't use affiliate links.
The price of the Sennheiser HD660S2 is $/€600 and you can buy it from all authorized dealers around the world.



Sennheiser HD660S2

Below you will find some useful information about the design principles and the technological innovations of the HD660S2 as provided in the Sennheiser product brochure.
For the full literature you should visit the Sennheiser website.

"This warm, inviting signature combined with extended sub bass makes listening uniquely
pleasant and rewarding. And with comfort this good, it practically begs you to keep going
for endless sessions. That’s how HD 660S2 combines the best of many worlds to give you
an intimate experience you won’t want to live without.
– Natural, relaxed, high-fidelity sound with extra sub bass: Optimized surround and diaphragm with new voice coil with 300-ohm impedance
– 38mm high-performance transducers built in Ireland: Steel mesh damping follows the
driver’s geometry to control air displacement, while the magnetic yoke includes milled air
vents for optimal ventilation, which minimizes distortion
– Uniquely powerful vented magnet system for minimized distortion
– Ultra-light aluminum voice coil with 300-ohm impedance for excellent impulse response
– Intimate sound with great timbre and deep, accurate sub bass

Frequency response 8 Hz - 41,500Hz
Impedance 300 Ω
Sound Pressure Level (SPL) 104dB(1 V)
Total Harmonic Distortion (THD) < 0.04% (1 kHz, 100dB)
Weight 260g



According to Sennheiser, the main differences between the two headphones can be summarized as:

Lighter voice coil by ~10%
Lowered resonant frequency of surround vs mk1
Retuned transducer
Now 300 ohms; higher magnetic force vs mk1 (150 ohms) for speed and blacker backdrop
Same steel mesh baffle damping, same mesh ear cup outer grille
Same ventilated magnet design with better airflow (but less air leakage vs mk1 for punchier dynamics in bass range)
Smoother top end vs mk1, with increase in clarity that feels more natural.



Appearance and fit

The Sennheiser HD660S2 is identical looking to the HD660S and the only discerning difference is the bronze color of the side logos instead of the silver of the original model.
The headphone is entirely made of high quality plastic except the steel spring of the headband and the aluminum outer grills.
Build quality is excellent for this type of construction but it can't match that of some other headphones like the Meze 109 PRO which is also more luxuriously looking and built with higher quality parts.
The ear pads are made with the same velour outer material as with the whole HD6xx series and they are comfortable but don't have enough room to house the whole ear while they get quite warm after a while.
The overall feeling is that of a comfortable headphone which is additionally very lightweight, making it suitable for extended listening sessions minus the heat.
The initial clamping force was medium, it suited my head shape well and never felt too strong but your experience may vary.
You can always carefully bend the headband and adjust the clamping force to your liking.
All headphone parts are user replaceable and can easily be sourced from the Sennheiser retailers.



Accessories

The headphone comes with a simple carrying pouch, two 1.8m cables with 6.35mm and 4.4mm plugs and a 6.35mm to 3.5mm adapter.



Associated gear and power handling

The Sennheiser HD660S2 features a new 300Ω driver with a sensitivity of 104dB(1V), almost the same as the HD650 (103dB), so it is really not that difficult to drive as long as you have an amplifier capable of providing a good voltage swing.
I have tested the headphone with various sources in order to form a better and more comprehensive opinion about it being difficult or not to drive.
The FiiO K7 and K9 PRO (balanced outputs), Schiit Lyr+ and Vali 2, Violectric V380² (balanced output), Feliks Audio Euforia, SMSL HO200 (balanced output) and Topping DX5 from desktop gear while the FiiO M11 Plus ESS, M17 and iBasso DX320 (AMP12) where used as portable sources (balanced outputs).
All of them provided excellent driver control and plenty of headroom with all kinds of music and no one of them felt as lacking in power.
The headphone scales very well and will highlight higher quality electronics but up to a certain point as it cannot eventually compete with really high end headphones, like the Meze Elite and HiFiMan Susvara, in scaling potential.
Still the Sennheiser HD660S2 is sounding like a more expensive headphone, outperforming other competitive headphones, like the Meze 109 PRO or the HD660S when it comes to source scaling.



As expected, the high impedance driver pairs very well with OTL tube amplifiers, like the Feliks Audio Euforia, and the listener is rewarded with a full and high fidelity sound but you shouldn't expect any tonal shifts at least when using a source with a linear frequency response.
Another thing of note is that the new driver can handle louder listening levels better than the HD660S because of the very low distortion.



Listening impressions

The HD660S2 was burned about 100 hours before listening evaluation.
A sound comparison between the HD660S2, the HD650 and the HD660S is included in the course of the main review and not in a separate section.
All headphones were used with the stock ear pads, brand new for the HD660S2 and like-new for the other two.
The HD650 is modded with the Custom Cans, copper - weight mod that only affects bass technicalities without altering the frequency response.
I have used both the stock and Lavricables Ultimate pure silver cable.



This is the first HD6xx series headphone with real sub-bass extension and not merely harmonics that pretend to act like one.
It goes deep without being boosted but the frequency response in not truly linear, there is still an audible mild roll-off, nonetheless it can now clearly reproduce low pitched instruments, like the double bass or the pipe organ while it can sound very satisfying and extended with electronic bass but don't expect bass-head levels of boosting.
The bass as a whole is relatively balanced but not strictly neutral and reference tuned, the HD660S2 is following the route of the HD650 with a mildly emphasized bass and upper-bass.
The main difference is that now with the actual sub-bass extension the headphone achieves a low end tonal balance and equilibrium and it doesn't give the impression of the detached mid-bass hump like it did in the HD650.
The transition to the mids is even, the bass doesn't bleed into the region nor sound as overlapping them.
On the other hand, the HD660S bass tuning is more neutral, linear and reference-type but unfortunately it is lacking in real sub-bass extension.
Comparing the two of them while listening to classical music, let's say the second movement of Shostakovich's 5th symphony, there are take-aways for both headphones.



What happens is that certain low pitched instruments, like solo bassoon or cello, will sound more "in tune" and realistic with the HD660S when the S2 will add an unnecessarily coloring but you will find the S1 is lacking in intensity and extension with various instruments like low basses or timpani that sound a little muted and not as deep reaching.
Technical performance is really amazing, the bass is tight, layered and controlled with plenty of texture and excellent definition.
The S2 is a touch warm and cozy sounding but not dark or veiled, it is way more cleaner and transparent than the HD650 and considerably faster, you can hear every last note and feel the rhythmic pace of the bass line.
They trade blows with the HD660S in technicalities but the S2 is considerably more visceral and full bodied compared to the leaner and drier S1.
There is also a notable improvement into rendering the macro-dynamics, the new driver is punchy and impactful with excellent rise and fall times, it sounds very contrasted and powerful, not to the Focal Clear Mg levels, but this is undeniably the best performer of the 6xx family and maybe even better than the HD800S.

The mid-range has a character and a flavor of its own that differentiates it from the famous HD650.
The S2 has a balanced, almost linear, lower mid-range that is followed by an emphasized center mid-range while it steers away from the upper mids boosting of the HD650.
A very forward, mid-centric tuning that clearly favors human voices, especially contralto, male alto and soprano and gives an extra projection to certain instruments or their harmonics.
Voices get always highlighted and projected in front of the listener who additionally feels very closely positioned to the performers, a type of tuning that might work very well for a lot of people but others might find it fatiguing and tiring in the long run.
When listening to the famous male alto Philippe Jaroussky with the HD660S2 and the HD650 you will find out that his voice sounds more natural and realistic with the latter than the former and additionally the listener feels like actually sitting and listening to the performance instead of hugging the singer.
The HD650 is more sweet sounding and rounded with some extra lushness and elasticity to the mid range compared to the more aggressive and enthusiastic HD660S2.



The second movement of Rachmaninoff's second piano concerto opens with a series of slow chords in the strings, then the piano enters playing a simple arpeggiated figure and then the main theme is initially introduced by the flute, before being developed by an extensive clarinet solo.
With the HD650 and the HD660S the woodwind sound was more balanced and naturally integrated with the strings and the piano when with the S2 got mildly emphasized and artificially highlighted.
On a technical level, the mid range of the HD660S2 is very transparent, crystal clear and finely articulated with plenty of harmonic variety, natural overtones and a full bodied texture.
The timbre is very realistic, the music gets emotionally connected with the listener who is rewarded with an engaging, organic and realistic listening experience.



The transition to the treble is more smooth than the 650 and the S2 avoids the upper treble peak that many people found as irritating and harsh sounding.
The treble as a whole is really exquisite, the engineers have achieved a tuning that is full of energy and light, extended and resolving yet smoother and with a more easy going nature than the HD660S, you can listen for hours long without getting fatigued.
This is a treble of the highest quality and fidelity, the S2 sounds airy, crisp, crystal clear and ultimately transparent.
Additionally the texture is weighty and rounded, not as dry and thin as the HD660S while it decays in a more relaxed way helping instruments to sound full bodied and realistic in their reverberation in time and space.
The detail retrieval is phenomenal for the category, the driver offers a pitch black background that helps in extracting all the finest particles of the music and adding great micro-dynamic contrast without sounding analytical or artificial.
Competitively, the HD650 is sounding veiled and hazy while the HD660S offers a touch more air and sharpness in the treble but can become a little hot and aggressive in this region.



The soundstage is on the HD650 level regarding width and spaciousness but now it feels more intimate, it positions the listener extremely close to the action, sitting together with the performers rather in front of them and the end feeling is that the music is emanating from the insides of his head.
So we are talking about a relatively narrow and claustrophobic soundscape with a strong center image but with a very sharp, precise imaging and good depth layering so as not to sound congested or one dimensional.
Still the HD660S fairs better than the rest of the family with the more spacious and airy soundstage that is slightly wider and more open while it places the listener in a more realistic and proportional distance from the performers.
Listening to large symphonic works with choruses is more rewarding with the spaciousness and openness of the HD660S than the HD660S2 where the proximity to the performers doesn't sound realistic while despite the excellent positioning you have to be really concentrated in order to pinpoint every instrument.



Compared to the Meze 109 PRO $799

The Meze Audio 109 PRO is $200 more expensive than the 660S2 but in exchange you get a more luxurious appearance, higher quality and more premium materials, better build quality and a hard carrying case.
The headphone comes with two cables, 1.5m and 3m length, but they are both single ended, you have to buy an extra one if you want to go balanced.
The 109 PRO is more easy to drive and can be used with lower power output portable gear.

The comparison was made with the FiiO K7 and iBasso DX320 from their balanced outputs and with the Lavricables Ultimate pure silver cable.
Volume level was roughly matched by ear.



The 109 PRO has a real sub-bass extension that doesn't roll-off at all so it sounds deeper and fuller especially with electronic bass tunes and modern music
Then the bass is even less neutral than the HD660S with a linear but well emphasized response up to the lower mid-range.
This is a tuning with plenty of bass and mid-bass that favors the fun and enjoyment factor rather than actual tonal accuracy.
It is not that you can't listen to classical music but the instruments sound slightly out of tune and emphasized compared to, the already slightly colored, 660S2 which comparatively is more in-tune sounding.
The 660S2 has slightly better technicalities, it is a bit more tight, layered, better defined and controlled but also more drier and not as full bodied and visceral as the 109 PRO.
Both headphones are equally fast, the 109 PRO has the lead when it comes to dynamics, impact and raw power but it can also sound a little hollow and bloomy.
The bass on the 109 PRO will just slightly bleed into the mids but it can't be said that it is overlapping the rest of the frequencies or clouding the mid-range.
The 660S offers a more even and linear transition into the mids with better overall clarity.
The center mid-range of the 109 PRO is more conservatively tuned, slightly reserved compared to the emphasized 660S2 so it offers a better tonal balance and it doesn't sound as pronounced and at-your-face, voices are less prominent and better blended.
Then the two headphones follow opposite directions, the 109 PRO has an upper mid-range and treble emphasis compared to the subdued 660S2 that results in a more lively, forward and sharper tuning.
The 109 PRO is more fresh and youthful sounding with an abundance of energy and plenty of luminosity but is sometimes touching brightness territory.
It is not harsh or fatiguing but it is not as smooth and controlled as the 660S2 while it has the tendency to sound thinner, a little metallic and artificial with a rushed decay over time so percussion instruments don't sound that naturally shimmering.
The 660S2 is more resolving and refined with a treble texture of higher quality that responds better in upstream gear and offers finer micro detail retrieval.
As a whole the timbre on the 660S2 is more natural and realistic throughout the whole frequency range, this is a more serious sounding headphone when the 109 PRO is more youthful, fun, dancing and partying.
One notable difference between the two is the soundstage which is more extended and open sounding in the 109 PRO with extra spaciousness and air around the instruments while it places the listener at a more proportional distance from the performers than the very intimate sounding 660S2.
The 660S2 has a more centered image than the more diffused 109 PRO.
An interesting comparison is that when listening to solo singing voices, like an opera aria, the voice on the 660S2 is projected close to your face and at the center of it, at the front of your nose, when with the 109 PRO it feels as being moved a couple of meters away and at the upper part of your head, at your eyes height.
Sub 1K region is becoming more and more competitive with some headphones that offer great performance for the buck like the two discussed here.
Don't ask which one is better, this is something that you must decide after carefully considering your listening habits.


In the end

So this is the new kid on the block and before even publicly released the debate has already started whether it is an upgrade, a downgrade, a side-grade or if it is the real successor to the HD650 or the HD660S etc.
As always the answer cannot be simple and one-dimensional because there are a lot of parameters involved.
Outside the Sennheiser ecosystem, the HD660S2 is an undeniably excellent sounding headphone with supreme technicalities and a balanced, realistic tuning that offers plenty of musicality and emotional involvement and as such it directly compares with similar or slightly more expensive headphones like the HiFiMan Ananda Stealth or the Meze 109 PRO.
There is no right or wrong here and as always, listening habits and preferences will help you to choose the right headphone for you.
Back to the Sennheiser family, the HD660S2 is definitely an upgrade because it offers the missing sub-bass extension and it has a more modern and technically capable driver.
But then it has a slightly different tuning than its older brothers which is not necessarily better or worse, it is just new and different.
So, there are great chances that you are going to like it and crown it as the successor of your current HD6xx headphone but there are also equally great chances that you are going to stick with the more neutral bass response and open soundstage of the HD660S or the clearly distinctive and unique mid-range tuning of the HD650.
The HD660S2 is essentially a new variation on the same theme and while this might not sound very exciting we better remember that in music sometimes the variations are actually of higher artistic quality than the original theme.
And Sennheiser, acting as a skilful and inspired composer, has managed to create a little gem variation that, in my opinion, has managed to surpass the original theme.

Test playlist

Copyright - Petros Laskis 2023.