INTRODUCTION
Itâs time to come back down to reality after a brief (but fun!) foray into high-end headphone systems. The Sennheiser HD660S headphone crossed my desk recently and I had a good listen. Letâs see what I foundâŠ
TL;DR
The Senn HD660S is a very good ~$200-250 headphone that unfortunately sells for $500 from most retailers. It has its strengths and offers a slightly different signature than its popular sibling model the HD6XX from Massdrop. Itâs more forgiving than its 6XX sibling of entry-level DACs and amps. But, in this reviewerâs humble opinion it does not offer a signature that is different enough or a technical performance improvement over the 6XX, or another budget heavyweight Beyerdynamic DT880 600âŠ, large enough to justify its ~$300 price difference over those models. However, if you can find it used around $250 or less, it can be a compelling option.
KNOW YOUR REVIEWER
My preferred genres are rock/metal and classical/orchestral music. Iâm getting to know jazz more and enjoying quite a bit. I also listen to some EDM and hip-hop. My hearing quirks include a high sensitivity to midrange frequencies from just under 1KHz to around 3Khz, give or take. My ears are thus quick to perceive âshoutinessâ in headphones in particular. I describe âshoutinessâ as an emphasis on the âouâ sound of âshout.â Itâs a forwardness in the neighborhood of 1KHz and/or on the first one or two harmonics above it (when I make the sound âooooowwwwwâ into a spectrum analyzer the dominant frequency on the vowel sound is around 930Hz, which also means harmonic spikes occur again at around 1860Hz and 2790Hz). In the extreme, it can have the tonal effect of sounding like a vocalist is speaking or singing through a toilet paper tube or cupping their hands over their mouth. It can also give instruments like piano, but especially brass instruments, an added âhonkâ to their sound. I also get distracted by sibilance, or sharp âsâ and âtâ sounds that can make ssssingers sssssound like theyâre forssssssing esssss ssssssounds aggresssssssively. Sibilance does not physically hurt my ears nearly as quickly as shout, though. Itâs distracting because itâs annoying and unnatural. Finally, in a new clause in this section, Iâm discovering that I have a preference for more subtle detail. I like good detail retrieval and hearing what a recording has to offer, but I prefer that presentation to what many would consider relaxed and subtle rather than aggressive of detail-forward. To my ear, more subtle detail-retrieval sounds more realistic and natural than aggressive, detail-forwardness. There is a balance here, though, because detail retrieval can get too relaxed and that can sound unnatural, as well. Readers should keep these hearing quirks and preferences in mind as they read my descriptions of sound.
FEATURES & BUILD
The HD660S is an open-back, dynamic-driver, over-the-ear headphone. It has a rated impedance of 150⊠and rated sensitivity of 104 dB/mW. On paper, itâs an easier-to-drive load than most of the Sennheiser HD6?? series, which have impedances of 300âŠ. As far as build quality goes, thereâs not much to say here that hasnât already been said. If youâre familiar with the Senn 600 series build, thatâs what you get. Iâm not going to spend too much time here because descriptions of the build are all over the internet. The time here will be better spent on theâŠ
SOUND
Test Gear
There were 3.5ish different signal chains I listened to the 660s with most. To acclimate, I used the Chord Hugo 2 DAC/amp (as DAC only) and the Cayin HA-1AMK2 tube amp on its lowest output impedance setting. Thatâs a source chain thatâs probably a bit unrealistic for most users who may be considering the 660s, so I also did a fair amount of critical listening with the Schiit Modius + Asgard 3 stack, and a Topping D10 + JDS Labs Atom and Monolith Liquid Spark amp. Read about some of my thoughts on Asgard 3, Atom, and Liquid Spark amps here.
Sound Signature
To my ear the signature of the 660s is a gentle âvââŠkinda. Why kinda? Thereâs not much subbass. The middbass is a bit forward, the mids are slightly recessed, and the lower treble is a bit forward. Itâs kind like a radical symbol: â - sorta, but the treble isnât noticeably forward of the bass. If youâre familiar with the HD6XX, pull back the mids a touch and bring the lower treble up just a hair. The result is not as mid-focused or overall neutral-warm like the 6XX is, itâs a bit closer to a more mainstream tuning, but not nearly as extreme. Itâs bass and treble extension are both somewhat mediocre, not going very deep or having much air, but thatâs par for the course for Senn 6?? series cans. And like the other Senn 6?? cans, the 660s is generally smooth, relaxed, and easy to listen to for long periods of time without getting fatigued.
Detail Retrieval
Have you heard the phrase âSennheiser veilâ? Senns are not detail-forward so to many they can sound veiled when they are first put on. It took me awhile to drop this feeling after doing weeks of listening to the likes of the Audeze LCD-24, HiFiMan HE1000V2, and Abyss Diana Phi. However, Senns are actually quite detailed at their price points; theyâre just subtle about it. Once that veiled feeling wears off, thereâs a fair amount going on. The 660s is no exception here. Itâs reasonably resolving in the mids and lower treble in particular. Room reverb, many vocal subtleties, etc. are presented well.
Spatial Presentation
The spatial presentation is still very Senn HD6??-like. Itâs not particularly wide, its imaging tends toward the â3-blobâ presentation (stuff left, stuff center, stuff right, not as much in between as some others), and thereâs not a ton of depth. None of those things are deal breakers for most, so they wonât be here either. The 660s does a nice job of creating some more vertical space than some other headphones Iâve heard in the price range, though.
But If You Tube It
Yeah, itâs a Senn 6?? series, so we gotta talk about tube amps. At 150âŠ, the 660S is basically the minimum impedance that makes sense to use on an OTL (output transformer-less) tube amp. Unfortunately, I donât have a true OTL tube amp, I have the transformer-coupled Cayin HA-1AMK2. The 660s did indeed sound better on the 1AMK2 than it did on the Atom, Liquid Spark, or Asgard 3, but I donât think that had much to do with it being a tube amp and the 660s taking well to tubes. If anything, itâs because the 1AMK2 is on a much higher performance tier than any of those amps. The truth is, the 660s did not change its sound much. Yes, there was slightly more detail and an improvement in overall technical performance, but there wasnât the transformation that happens when the 6XX is put on a tube. So, you can use the 660s with a tube. It does change a bit. But, itâs not a completely different headphone on a tube in the way that its 600 and 650/6XX brethren are.
The 660s is a fine-sounding headphone. I can listen to it and enjoy it for extended periods of time. Nonetheless, this sound section is a bit shorter than I often do because I think the big story with the 660s is found in theâŠ
COMPARISONS WITH OTHER HEADPHONES
I keep a Massdrop + Sennheiser HD6XX (which is essentially a Massdrop-branded HD650) and a Beyerdynamic DT-880 600⊠around as references pieces. Both of these cans are near $200 USD in MSRP and both are well-known, popular cans. They make excellent references for review work. Iâll go ahead and drop the punchline of this section here: the 660s has its own sound signature that may attract some listeners but it also is not a marked technical improvement over the 6XX or the DT880 â if itâs even an improvement at all. It also doesnât scale up nearly as well. Both the 6XX and DT880 have something new to give when theyâre driven by $1000+ amps and DACs. The 660s hits its performance ceiling much earlier. On the flip side, I found the 660s to be slightly â and I emphasize slightly â more forgiving with entry-level gear than either 6XX or DT880. I interpret this to mean it is a little less source-picky than these other two. Alright, letâs unpack all of thisâŠ
Sound Signatures
As stated before, the 660s has a very mild and somewhat rolled-off-at-either-end v-signature. The 6XX has a neutral-warm signature that is also somewhat rolled-off with mediocre extension both high and low. The DT880 is neutral-bright with excellent bass and treble extension but also somewhat lean bass. The amount of treble in the 660s is between the 6XX and DT880, but closer to the 6XX than the 880. The 660s and DT880 are quite similar in midrange quantity. The 6XX and 660s have similar amounts of midbass, being slightly more than the DT880, but the DT880 has the extension advantage and more subbass presence overall, despite it being somewhat lean in the lows (at least compared to its treble). I think the DT880 also has more bass punch/slam. Itâs more dynamic. I wouldnât say it has great punch/slam because of the lean-ness of its bass, but it is overall more energetic and aggressive throughout the frequency range than either Senn.
Spatial Presentations
The DT880 is the spatial king at its price point and its abilities in this area keep it competitive way beyond its price point. Itâs well ahead of either Senn model in soundstage size, imaging, separation, and depth. After that, and in a somewhat shocking discovery for me given the intimate nature of the 6XXâs spatial presentation, the 660 is more horizontally narrow than the 6XX. Itâs not a huge difference, but itâs noticeable. The 660s is also actually more 3-blob-y than the 6XX â another shocker. However, as mentioned before, the 660s handles vertical space much better than 6XX, creating a much more convincing sense of soundstage height.
Detail Retrieval
The DT880 initially comes across as the most detailed of the three because of its brightness and more aggressive presentation. Upon closer listening, it maintains a slight edge over either Senn in detail retrieval in the bass and the treble. The 6XX and the 660s are slightly more detailed in the midrange than the DT880. The 6XX probably is the one with more resolution in the mids, and the 660s is the one thatâs smoother and more forgiving. Both of these qualities have advantages in certain situations.
When To Use These Headphones
The 6XX and DT880 both are a bit on the picky side. The 6XX is very transparent in the midrange and will wander into shouty/honky territory if its source chain (amp and DAC) arenât strong in the mids. I learned this the hard way the first time I owned a 6XX. I was using a DAC that didnât do so well in the mids from its single-ended output and that translated into shouty and honky mids, and was not particularly enjoyable. The DT880 has similar challenges in the treble. On a source chain that doesnât handle treble very well, it will get sharp and shrill to the point of piercing sometimes. I did not notice such issues with the 660s. It sounds pretty good â probably the best of these 3 headphones â on the ~$75ish (and ESS-based!) Topping D10 DAC, and Atom ($99) or Liquid Spark ($109) amps. The midrange is smoother and more natural on that stack with 660s than the 6XX and the treble is pleasant and reasonably detailed where the DT880 can get over-the-top sharp (especially on Atom).
Moving up to the Modius + Asgard 3 pile of Schiit, the 3 headphones are on a roughly equal technical level across the board and signature preference will make the determination as to which one is the favorite. On this stack, the DT880âs sharpness is largely tempered and the 6XXâs mid-shout is virtually absent. IMO, the Asgard 3 in particular is the entry-point for when the 6XX and DT880 should be reasonably considered. The DT880 can sound pretty good on the Liquid Spark most of the time, but still gets a bit shrill even on that warmer amp. With the Asgard 3 its treble sharpness will be a non-issue for all but the most treble sensitive listeners. The 6XXâs mids really need the warm, steady hand of the Asgard 3, though. The 660s is quite enjoyable and competent. It has its own signature that some will find appealing. But, at $500 itâs not yet separating itself in technical proficiency from either of the other $200 headphones.
Itâs when the source chain moves into the (multi)kilobuck range where it becomes difficult to justify the 660s costing $300 more than 6XX or DT880. On the Hugo 2 + HA-1AMK2 combo, the 6XXâs mids sound very detailed and organic, with stunningly natural timbre considering the price point. The DT880 musters hints of meaningful bass texture and has treble timbre every bit as good as what the 6XX delivers in the midrange. The 660s sounds better than it did on the $400 Schiit-pile but does not sound as technically proficient as the 6XX or DT880 at this level. It lacks a standout feature that the other two bring.
What does this mean? Itâs tricky. My read on this is the 660s is a mid-fi-priced headphone that should be priced more in the $200-250 range. It would be a solid headphone at that price and wow many audiophile newbies. While Iâve not heard it, my understanding is the Massdrop + Sennheiser HD58X kinda is the 660sâŠalmostâŠat $185. The prices of the 6XX and DT880 are also misleading. They are priced as entry points into the headphone-enthusiast game. But they require a healthy investment in the electronics that drive them to hear what makes them so special. They are mid-fi headphones priced at the entry-fi (for enthusiasts, anyway) level. Some of that pricing is because theyâve been around for a long time and so paying for development cost is no longer necessary. The Senn HD650 was $500ish once upon a time, and so was the DT880 long ago. [The core DT880 design has been around since 1980!] Nonetheless, the 660s is a fine-sounding headphone that seems to be struggling for a true place in the market. Yes, itâs signature is different, but not that different. Itâs a fine headphone that doesnât really have a niche, IMO.
And then thereâs tubing. A great party trick the 6XX has is that itâs essentially 2 headphones. It has one signature for solid state amps â which is pretty good â and then really wakes up on a tube amp with more alive bass and treble. The DT880 isnât quite as dramatic but also changes its behavior significantly between solid state and tube amplification. The 660s doesnât change as much as either of these other two. Does it sound good on tube amps? Yes. It does. It sounds good on solid state amps too. It just doesnât transform to the degrees that other Senn 6?? do and doesnât match the difference the DT880 gives either.
FINAL THOUGHTS
The above probably makes it sound like I donât like the HD660s. Thatâs not true. I genuinely enjoyed its sound when I listened to it on its own merits. I spent several dozen hours with it before comparing it against the HD6XX or the DT880. I enjoyed that time. I really did. I was pleasantly surprised. But then I did put on the HD6XX and DT880. And every time I go back to one of those two I think âwhy donât I listen to these more?â The effect is not the same with the 660s because it doesnât scale up like 6XX or 880. At that point I started looking at prices and numbers. Itâs a total of approximately $675 for the HD660s, Topping D10 (now D10s), and JDS Labs Atom. Itâs $620 for Schiit Modius, Schiit Asgard 3, and HD6XX or $600 if you swap the DT880 for the 6XX. The experience with the Schiit stack and 6XX/880 is better than 660s + D10 + Atom. The experience with the 660s + Schiit stack is about the same in overall quality as with Schiit stack + 6XX/880 but is also now $900 instead of $600ish. At that point itâs more cost-effective to get the Schiit stack + 6XX/880 and another complementary headphone for $300. And then either the 6XX or 880 will stay relevant in your collection several upgrades down the road when this hobby inevitably leads you to thousand-dollar or more amps and DACs. The 660s will run out of tricks up its sleeve.
For me the bottom line is the 660s is an enjoyable headphone that makes more sense if you can land it for ~$250. Even $300 is pushing it, IMO. It has a different signature than DT880 or its HD6XX sibiling, but not different enough in my view to justify $300 more for it. Itâs an enjoyable headphone that appears to struggle to make its own unique mark on the market and justify its $500 price tag.
Thanks for reading another long review, all. Enjoy the music! 
