Written by @Chrono
Review unit provided on loan for review by headphones.com
The HD 600-series from Sennheiser is, without a doubt, one of the most successful audiophile headphone lines ever created. The HD 650 and HD 600 have been around for almost two decades, but they continue to be a favorite amongst listeners. With its addition to the 600-series family in 2017, does the HD 660S live up to its legacy, and is it worth its $499 price tag?
Sources and Music Used in Listening Tests
The Amplifier/DAC used in this review was the JDS Labs Element II connected via USB to my desktop computer. For the listening tests I used music from a wide variety of genres including Rock, Jazz, Classical, Acoustic, Hip-Hop, and Latin. I played tracks from my own FLAC library as well as from Qobuz streaming service.
What’s in the Box?
The HD 660S’ packaging is definitely on the more minimalistic side, but it does include just about everything you need to start listening. Included with the HD 660S are two cables; one has a ¼” termination, and the other one is 4.4mm balanced. I am not usually one to complain about cables, but I did want to mention that the included cables are both very long (3m) and they are not particularly easy to wrap up–they are quite unruly. Lastly, Sennheiser also includes a ¼’ to 3.5mm adapter.
The HD 660S comes in at an impedance of 150ohms and a sensitivity of 104dB. I personally found them to have plenty of volume on most devices I tested, like my MacBook Air and Nintendo Switch. However, I did think that they sounded better when being fed power from my amplifier. When running them on devices without a discrete amplifier I thought that they sounded a little more compressed, so I will list an amplifier as recommended for these. Still, these are very efficient, so it does not have to be a very powerful amplifier. Amplifiers like the Liquid Spark and JDS Labs Atom will be more than enough to drive these very cleanly.
Build Quality and Comfort
The HD 660S rocks the traditional HD 600-series chassis but with a slick all-matte-black finish. With the exception of the outer grills, and inner headband, the HD 660S’ body is composed almost entirely out of plastic. Still, this is one of the most durable, and reliable headphone designs I have seen on a headphone. I have owned a couple of these Sennheiser headphones over the years, and not once have I had issues with their build quality. The only part on this headphone that will need replacing is the pads, which wear out extremely quickly. You will probably have to swap the pads out every year or so to ensure that they continue to deliver their intended sound.
Like the build quality, comfort is also very good on the HD 660S. Thanks to their mostly-plastic construction, these are extremely light (only 260g or so), so I doubt anyone will have weight issues with these. The pads use a fairly dense foam wrapped in velour and are very soft on the skin, I only wish that they were a bit wider on the inside as some people’s ears might touch the inner sides of the pads. The only problem I have with the comfort is that out-of-the-box, the clamp force can be quite vicious. Nonetheless, this can easily be alleviated by extending the headband all the way out and gently flexing the part that extends.
Right away I will say that these are my favorite headphones in Sennheiser’s HD 600 line-up, and for the music that I listen to the most (classic rock), I really, really enjoy them. However, they are definitely not perfect, and–I think more importantly for the HD 660S–there is the question of value; which I will discuss towards the conclusion of this review.
For this sound section I will be going over the HD 660S’ tonal and technical performance while drawing comparisons to some of its competitors, as well as some of Sennheiser’s other offerings; namely the DROP X Sennheiser HD 6XX and HD 58X Jubilee.
Without a doubt, the bass region is where the HD 660S feels the most lacking. It actually has very good speed, it really is not all that far behind the DT 1990 Pro for detail in the lows. I also think the bass on these is more articulate than on the HD 6XX and 58X. However, where it really lags behind is in extension. The bass on the HD 660S rolls off aggressively starting at around 80hz, so you definitely will not be getting much sub-bass in this headphone. This shallowness in the bass is a very common trait amongst the HD-600 series headphones, and any improvements the HD 660S has for extension over its predecessors are negligible; they have nowhere near the level of depth that competitors like the HiFiMan Sundara, Audeze LCD-1 and DT 1990 can reproduce. The HD 660S also has a very small elevation at around 150hz; and while it is not not super noticeable, I did think it could overpower some of the frequencies that precede it, taking away some definition in bass-heavy tracks.
As expected from an HD 600-series Sennheiser, the mids on the HD 660S are simply spectacular. In terms of frequency response, they are simply perfect to my taste. The upper midrange on the HD 6XX and HD 58X came across as a little forward or shouty, as they had a little extra energy at around 3k-3.5k, but this was not the case at all for me on the HD 660S; I found its upper midrange to be very smooth. The timbre on this headphone’s mids is–again, like the other headphones in the series–spot on and very natural sounding. For detail retrieval and resolution in the midrange, I thought that these performed great as well. From my listening, I personally found them to be an upgrade over its predecessors as well as a little more resolving than the LCD-1, Sundara and DT 1990 Pro in the mids.
The highs on the HD 660S are definitely the brightest of the HD 600-series headphones, and while I would not call them veiled, they are definitely still on the warmer side. For frequency response I only had two minor issues with the HD 660S’s treble. The more noticeable one was a peak in the lower treble at 5.5k, that–like on the HD 58X–made the HD 660S’ timbre sound a little bit more congested than on the HD 6XX and HD 600. Additionally, the frequencies above 10k could use a little more energy for my preference, I thought that they sounded a little muted. Aside from that, I still really enjoyed the treble and I found it to be very smooth overall. For resolution I think that they have very good detail in the highs; they are very resolving and clean in their delivery. Comparing them to other Sennheiser headphones, I thought that for resolution they were a little more resolving than the HD 6XX, and were a significant upgrade in treble detail from the HD 58X. I personally thought they were more detailed in the highs than the DT 1990 Pro, and they were on par with the HiFiMan Sundara.
Soundstage and Imaging
Unfortunately, the soundstage on the HD 660S leaves a lot to be desired, as it is very narrow. You do not get much width in these; they are easily outperformed in this category by Sundara, and definitely by the DT 1990 Pro. Their ability to image and determine location of sounds is also fairly lacking. Whilst they do have a very strong left, right, and center image, there seems to be a slight gap at front-left and front-right. For music I think they perform alright, but for other applications like watching movies or gaming, there are definitely more immersive stages on other headphones. Despite the weak soundstage and imaging, I found the HD 660S’ instrument separation to be outstanding. The HD 660S performed really great when it came to its ability to distinguish the individual layers making up tracks in complex musical passages, and it did so almost as well as the Sundara, which I personally thought was very impressive.
I think that the HD 660S performs decently well for dynamics. The HD 600-series definitely is not known for being punchy, but I still thought that the HD 660S performed slightly better than its predecessors in this category. It was by no means the strongest out there, but the HD 660S did have some punch slam. I found microdynamics to be fairly well-defined as well, as I felt like all elements in the mix had a fairly good sense of tension and presence. Overall, I would say that the HD 660S perform well in this category, although I do think that other headphones like the Sundara and DT 1990 Pro make for a more engaging and energetic experience in this regard.
As I usually do, I have created an EQ profile for the HD 660S that seeks to bring it closer to my personal target, which is similar to the Harman target. Via EQ, it is very easy to adjust for that elevation at 150hz, but it is not really possible to alleviate the lack of extension as the bass starts to distort early when pushed, so I only add a very conservative 2dB shelf under 100hz. For the mids I do not adjust anything at all, as I think that they match preference almost perfectly. In the highs, I add a peak adjustment filter to correct the peak at 5.5K, which cleans up the lower treble and gets rid of the congested tone I mentioned. I also like to add a high shelf of 1dB to the frequencies above 10k as they were a little muted for my preference. If you would like to try out my EQ for the HD 660S, you can input these setting sinto your EQ software of choice
Low Shelf at 80hz, +2dB Q of 0.7
Peak at 150hz, -1.5dB Q of 1.4
Peak at 5500hz, -5.5dB Q of 5
High Shelf at 11000hz, +1dB Q of 0.7
I mentioned earlier in this review, that I considered value a very important factor when talking about the HD 660S. The reason for this is that, although they are a very good headphone, at their MSRP of $499 they are not as competitive when compared to some of the other options currently in the market. I think mainly of the HiFiMan Sundara and DROP X Sennheiser HD 6XX which greatly undercut the HD 660S in price while delivering tonal and technical performance parity or something close to it. The HD 6XX is available on DROP at $220, which is roughly half the price of the HD 660S’ MSRP. At $220, the HD 6XX achieves what I would consider to be a very close experience to that of the HD 660S. Then there is the Sundara, which at $349 has slightly better soundstage, significantly better bass, similar level of detail and an overall tonal balance that I think will keep up a bit better across more genres. For those reasons, I find it hard to give the HD 660S a strong recommendation despite them being really enjoyable and well-performing headphones. Should the HD 660S come down in price to around the $399 mark, I think they become a much more compelling option as they are an excellent set of headphones that I sincerely believe most listeners will enjoy.
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