Sennheiser HD660S Professional Open-Back Headphones Review
Sennheiser has arguably achieved legendary status in the professional audio industry. They have been the go-to brand for both professionals and audiophiles all over the world. They are also the ones responsible for bringing products such as the Sennheiser HD600 and Sennheiser HD800S in the market.
The Sennheiser HD600, in particular, is arguably one of the most successful and influential headphones. It basically created the standard for modern audiophile headphones to follow. And even after almost two decades, the HD600 series is still recommended by purists.
But with the growing demands of modern users as well as the increasing number of competitors, Sennheiser has decided to update the beloved HD600 with the Sennheiser HD660S.
However, with its premium asking price, the Sennheiser HD660S becomes a question about value. Is the HD660S worth its asking price, or has the competition finally caught up with Sennheiser?
I have personally bought the Sennheiser HD660S at its retail price. We are not affiliated with Sennheiser in any way. All thoughts and opinions are my own.
Packaging and Unboxing Experience
Sennheiser delivers the goods and presents the HD660S in a very premium package. Its box is separated into two pieces. You get an outer box that contains the product photo as well as the technical specifications.
The inner box contains the fancy Sennheiser HD660S soft storage, which can also double as a carry case. Opening it reveals the headphones in all of its glory. The other accessories such as the cables and the paperwork are all neatly packed inside their own compartments.
The HD660S unboxing experience certainly gave me goosebumps. This alone reflected the HD660S’ premium status that is painted all over their website and promotion material.
One complaint that I have with the HD660S is that it doesn’t come with too many accessories. Aside from the cables, which we will be getting into later, there aren’t any bonuses with the HD660S. This doesn’t come as a surprise since pretty much all of their headphones are like this.
However, competitors are now offering carrying cases, extra earpads, and other accessories that help bring the overall package to the next level. I wish to see more accessories in Sennheiser’s next premium headphone release.
Design and Build Quality
The HD660S is pretty much the exact same headphones as the rest of the HD600 series when it comes to its design and build. Of course, this isn’t a bad thing. After all, the HD600 is an iconic pair and is usually associated with the modern audiophile look.
Additionally, the build quality of the HD600 series is revered in the community. They are made of plastic which is generally not the greatest material. However, they are so well made that most pairs are known to survive for decades.
And with how minimal the changes are to the HD660S, you can expect them to last for a very long time. And if you run into any issues, Sennheiser offers replacement parts for any part of the headphone.
There are some notable changes that help differentiate the HD660S from the other HD600 headphones. The HD660s now comes in an all-black color scheme that helps give it a more modern look.
Compared to previous iterations, branding is kept to a minimum. The only ones that are visible can be found on the left side of the headband and on the grills.
Unlike previous releases, Sennheiser now includes two stock cables for the HD660S. You get an unbalanced cable that terminates in a 6.35mm quarter-inch jack as well as a 4.4mm pentaconn balanced cable. You also get a quarter-inch to 3.5mm converter for the 6.35mm cable, which is great to see for those who do not use a quarter-inch jack.
Unfortunately, the quality of the cables is still not that great. They are sturdy enough for professional applications. Mine hasn’t encountered any issues in the two years that I have been using them.
However, I simply cannot stand the length of these cables. They tangle easily and are just overall disappointing for the price that you are paying. I highly recommend changing the stock cables in order to get a better listening experience.
The Sennheiser HD660s is one of the most comfortable pairs that I have worn. If you have tried any of the headphones in the HD600 series, you’ll know what this one feels like.
The earpads are also large and soft. My ears never touched the drivers. There was also no pressure buildup on my ears. This is one of the headphones that I can wear for multiple hours without needing to remove them.
Some users have, however, claimed that the initial clamping force out of the box is fairly tight. I personally did not experience this. But if you are one of those users, I highly recommend you to wear them for a few days. The clamping force eventually loosens up.
The sound signature of the Sennheiser HD660S is very similar to its predecessors. It has a neutral sound signature that is best suited for professional applications. But just like all other revisions of the Sennheiser HD600, the Sennheiser HD660S has its own characteristics that make it unique.
Most of the changes with the Sennheiser HD660S are aimed at helping modernize the HD600 sound. Sennheiser chose to update the weak points of the HD600 as well as upping the overall technical capabilities of the HD660S with the help of their newly updated drivers.
Of course, the signature midrange reproduction of the HD660s is back with the HD660S. We will talk more about it later. But without going into too much detail, the HD660S is the headphones to beat for those buttery smooth mid frequencies.
In this section, I will be breaking down each of the key frequencies and also talk about the performance of its imaging and soundstage. Also, most of the changes will be compared to the previous version, the Sennheiser HD650/Drop HD6XX.
The bass has been traditionally one of the biggest weaknesses of the HD600 series. Both the HD650 and the HD600 are often described as having no bass at all. This is, of course, an exaggeration, but that is pretty much how they fare against other open-back dynamic driver headphones.
But after comparing the HD660S to the HD650, I can confidently say that there is a significant improvement. The bass is tighter and hits harder. I also feel that the bass is capable of bringing in more detail, especially with instruments such as the bass guitar.
However, the HD660S is still lacking in bass extension, especially when compared to the likes of the Hifiman Sundara and Harmonicdyne Zeus. This does not necessarily mean that the HD660S will sound thin.
But the bass will surely not blow you away. They are meant to support the overall sound and will mostly take the backseat for most tracks.
I personally do not have issues with the bass while listening to my favorite tracks. The issue for me is when I use these for mixing. I almost always overdo instruments such as the bass guitar and the kick drum because I hear less lows than there actually are.
The party trick of the HD600 series makes its return with the HD660S. And this time around, the resolution and the overall quality are even better.
Listening to vocals and other instruments such as guitar is a treat with the Sennheiser HD660S. The mids just sound so natural and detailed that it’s hard not to have so much fun when listening to them. And when dealing with well-recorded vocals, you’ll easily hear the layering and the small details that are usually buried in a busy mix.
The only complaint that most enthusiasts have with the previous versions of the HD600 was that the upper mids sounded shouty. I did not find this issue with the HD660S. It was able to bring a smooth delivery that never sounded harsh, even with more intense vocal passages.
With how amazing these headphones resolve the mid frequencies, I do not think that its competitors can match its midrange performance. Of course, older HD600 models will be able to compete with the HD660S. But I feel that they don’t have the resolution and detail that the HD660S has.
The highs on the Sennheiser HD660S are a treat to listen to. They extend very well but are great at avoiding any sharp peaks or harshness. The result is an incredibly smooth experience that is able to easily resolve most instruments in this area.
And to my ears, the HD660S are not veiled. They are still on the warmer side and are certainly not on par with the brightness of the Sennheiser HD800. And overall, I did not find the detail retrieval to be lacking.
Even when the mix got incredibly busy, the highs did not lose their fidelity. Crash cymbals still sounded full and clear. Most headphones have a tough time with these kinds of sounds since crash cymbals usually sound messy in the mix. Overall, the highs are a clear improvement from the original Sennheiser HD600 and the Sennheiser HD650.
Imaging and Soundstage
The other issue that most users have with the Sennheiser HD600 series is its intimate soundstage. And while the HD660S has improved this aspect, it still won’t give you that speaker-like experience.
This does not mean that the HD660S sounds cramped. However, it surely isn’t the best representation of the open-back headphone experience. Even less expensive headphones such as the Harmonicdyne Zeus sound wider than the HD660S.
But with that said, I do not find the intimate soundstage to be a big issue. And that is because its presentation is fairly realistic.
Some headphones are known for exaggerating the soundstage and make them wider than they actually are. But with the HD660S, you get a consistent and natural listening experience.
I have tried playing some FPS games with the HD660S, and I did not feel that the soundstage made the experience worse. I would even argue that it helps me focus on other things. But of course, this will vary from each player.
Listening to orchestral tracks still felt wide and immersive on the Sennheiser HD660S. And part of the reason why it’s so immersive is because of its imaging.
The imaging on the HD660S is spectacular. I was able to accurately pinpoint all sounds with relative ease. All the small nuances on tracks are easy to hear.
In orchestral pieces, all the small details such as the positioning and the small differences of each of the instruments are noticeable. Even the panning effect became very obvious with the HD660S. This is certainly one of those headphones that bring already epic tracks up a notch.
Sennheiser heavily advertises that the HD660S can be powered by portable sources such as DAPs and generally less powerful sources. This was because the impedance was decreased from 300-ohms to 150-ohms. And while this is somewhat true, we believe that purchasing a more powerful source opens up the HD660S’ full potential.
We have tried pairing the HD660S with different devices. And we have concluded that the HD660S performs better when it is fed with more power. To prove this, we have tested the HD660S with the ddHiFi TC35B and TC44B, iFi Nano BL, Fiio M11, and Mass KOBO + Astell & Kern SP2000.
The HD660S worked with ddHiFi’s dongles. However, we were maxing out the volume, and we still felt like some details were missing. This is to be expected since these dongles are designed for IEMs and low impedance/efficient headphones.
Switching to the iFi Nano BL and the Fiio M11 helped give the HD660S a more open sound and tightened the bass. Both of these setups were already pretty satisfactory and should appeal to entry-level users.
But when we tried the HD660S with the Mass KOBO + AK SP2000, the overall performance was significantly increased. Its soundstage finally opened up, the bass was way tighter, and small details were easier to hear.
To sum up our little experiment, the Sennheiser HD660S scales well with better equipment. It may not have as much headroom as the older 300-ohm HD600 headphones, but it is still capable of sounding better with more advanced gears. We highly advise investing with a powerful source to get the most out of the Sennheiser HD660S.
When we are talking about competitors to the HD600 series, we usually think about models such as the Hifiman Sundara and Beyerdynamic DT 1990 Pro. But one model that we found to be an appealing alternative is the Harmonicdyne Zeus. And while it isn’t a professional pair meant for critical listening, we found a lot of qualities that were very competitive to the HD660S.
The Harmonicdyne Zeus improves upon the HD660S’ weak points. It has better bass and a better soundstage. However, it doesn’t outshine the HD660S in terms of the mids, highs, and technical capabilities.
The Sennheiser HD660S clearly edges out the HarmonicDyne Zeus when it comes to the mids. Very few headphones can beat the HD660S’ vocal performance, so this does not come as a surprise.
The HD660S has a more focused sound, while the Zeus’ mids were a bit more distant. And while vocals on the Zeus blend well in the overall mix, the HD660S takes vocals to the center stage and presents them with authority.
But with that said, the HD660S and the HD6XX series as a whole have some well-known weaknesses. These include the bass and the soundstage. These are areas where the Zeus beats the HD660S.
I find the Zeus’ bass to be a lot better both in terms of quantity and quality. They punch harder and are able to achieve a fuller sound compared to the HD660S.
The soundstage of the Zeus is also significantly wider. It manages to achieve this without sounding artificial. The imaging is also on par with the HD660S, which is very impressive considering the HD660S is well known for its accurate imaging.
The highs on both the Zeus and the HD660S are laid back and smooth. The HD660S has a slight edge with smoother and more detailed highs, but both are very close.
Overall, I would say the Zeus does a commendable job in competing against the HD660S. For my use case, I still prefer using the HD660S since it is more accurate sounding. But users who are looking for a more fun-sounding pair should consider the Harmonicdyne Zeus.
The Sennheiser HD660S is, without a doubt, the next evolution of the HD600. It takes everything that was good about the HD600 and HD650 and merges them to make a headphone that can compete with any modern professional pair.
But, given its price, the HD660S is surely not for everyone. Some may argue that lower-priced alternatives such as the Drop HD6XX and Drop HD58X can get close enough to the HD660S. And given the weaknesses of the HD600 series, some may even argue that other models such as the Hifiman Sundara can potentially give a more appealing sound.
Those are all valid points to consider. But for me, I think the HD660S is a worthy investment. They have the sound signature that I am looking for that suits both my music listening preferences as well as my professional work.
And its reliability and future-proof design are simply something that other brands cannot easily match. If you have the budget and are looking for a fantastic music production and music listening tool, I believe the Sennheiser HD660S is well worth the investment.
- Impedance – 150 Ω
- Connector – 6.35 mm / 4.4 mm Pentaconn
- Frequency response – 10 – 41,000 Hz (-10 dB)
- Sound pressure level (SPL) – 104 dB at 1V 1kHz
- Ear coupling – Over-ear (circumaural)
- THD + N, total harmonic distortion and noise – < 0,04% (1 kHz, 100 dB)
- Transducer principle (headphones) – Dynamic, open
- Weight w/o cable – Approx. 260 g (without cable)
Albums Used For Testing
- Milet – Who I am
- Babymetal – Legend Metal Galaxy
- Mamamoo – Travel
- Periphery- Periphery 3 and 4
- Blackpink – The Album
- Final Fantasy VII Acoustic Arrangements
- Square Enix Jazz- Final Fantasy VII At Billboard Live Tokyo
- Sawano Hiroyuki – Best of Vocal Works
- Yorushika – Plagiarism
- Intervals – Circadian
Stephen is a musician, cinematographer, and headphone enthusiast who is passionate about reviewing audio equipment. He has been playing guitar for at least a decade, which introduced him to professional recording equipment such as headphones and in-ear monitors. With the help of reviews and online content, he was able to learn the ins and outs of the hobby. His goal is to give back to the community by providing quality content to help others enjoy the beautiful (and expensive) world of audio.
Favorite Headphones: Sennheiser HD660s