The Sennheiser HD 660S is the successor to the much loved HD650, which is now being produced by Drop.com as the HD 6XX. While the HD660s is no longer a new headphone, within the context of its predecessor's longstanding history, the HD 660s' tenure as Sennheiser's higher-end version of this type of headphone has just begun. The general consensus has so far been that the HD 660s improves upon the HD 650, however its main detractor is that it's more than twice the price of the version produced by Drop, which comes in at just over $200. For many of us, the HD 6XX has been the first taste of truly great sounding headphones, and it's often considered the best place to start for anyone getting into this hobby.
Given the landscape of disproportionate pricing for entry to mid level headphones, I've been more curious about what the ideal next step is for anyone wanting to go a bit beyond the HD6XX. So far, my go-to recommendation has been the HiFiMAN Sundara (the current revision, which you can read about here). At $350, the Sundara improves upon just about every aspect of the HD 6XX, and so the question I've attempted to answer in this review is whether or not the HD 660s is worth the price jump from the Sundara.
Sennheiser HD 660s
- Driver Type: Dynamic
- Design: Over-Ear (Circumaural)
- Earcup Style: Open-Back
- Sensitivity: 104 dB
- Impedance: 150 Ohms
- Audio Connector: 6.35 mm / 4.4 mm Pentaconn
- Detachable Cable: Yes
- Weight: 9.17 oz / 260g
- Price: $499
- Driver Type: Planar
- Design: Over-Ear (Circumaural)
- Earcup Style: Open-Back
- Sensitivity: 94 dB
- Impedance: 37 Ohms
- Audio Connector: 1/8" / 3.5 mm Straight
- Connector to Earpiece: 2 x 1/8" / 3.5 mm
- Detachable Cable: Yes
- Weight: 13.12 oz / 372 g
- Price: $349
- iFi Pro iDSD -> Cayin IHA-6
- iFi Pro iDSD -> SPL Phonitor X
- iBass Dx220
- iFi iDSD Micro Black Label
- iFi Hip DAC
- Earmen TR-Amp
Build, Design & Comfort
Both headphones feel sturdy to hold, but while the HiFiMAN Sundara is made primarily out of metal, the HD 660s is all hard plastic. But plastic isn't necessarily a bad thing. In fact, the HD6__ line of headphones is well known for being long-lasting in spite of the plastic build. HiFiMAN on the other hand hasn't had the same track record, but I suspect much of the reputation comes from a previous generation of headphones that were notoriously flimsy. With the Sundara, I get the sense that HiFiMAN is trying to improve that reputation, and it shows with the more sturdy and rugged design. Still, time will tell as to which headphone will last longer, and if history has told us anything, it's that the longevity of Sennheiser headphones is great and HiFiMAN not so much.
Winner: Sundara feels more sturdy, but HD 660s has a better track record
For comfort, the Sundara is my pick here, but only because I find the clamp force to be a bit strong on the HD 660s, and I have a slightly larger than average head. I think for most people, the HD 660s' comfort won't be an issue, but I also find that the pads are a bit stiff - at least without any wear on them. This means that the parts of the headphone that actually come into contact with the head aren't as soft and comfortable as with the Sundara. The Sundara unfortunately doesn't have any swivel to the cups, and the HD 660s does have a little bit, so that's something to keep in mind. But the current iteration of the Sundara has pads that have a more extreme angle towards the front, meaning it doesn't clamp in as severely as the initial version. Both are acceptably lightweight, but the HD 660s is over 100g less, making it disappear more easily on the head for longer sessions.
The HD660s is a dynamic driver headphone and the Sundara is a planar magnetic headphone, meaning we have two completely different transducer types going on here. I was surprised to learn that the HD660s' driver is based on the one from the HD 700 because it sounds a lot closer to the HD 650 and HD 600 line. I suppose this goes to show how significant the rest of the design is for overall tonality. The Sundara's planar magnetic transducer wasn't borrowed from any previous designs, however this headphone has replaced the previous lightweight entry-level headphone in HiFiMAN's lineup, the HE-400i, and improves on every aspect.
Both headphones perform quite well in most areas, but it's important to remember that the HD 660s is $150 more expensive than the Sundara. It's also worth noting that there are noticeable differences between planar and dynamic driver headphones - enough that it can be difficult to properly compare them. But with that said, in this case I don't think having a preference for one driver type over the other should be the main focus for a purchase decision.
I also want to mention that both of these headphones require amplification, and they both scale with better gear. While I wasn't able to test them with high performance tube amplifiers, I did run the gamut of entry level to flagship solid state equipment, and the HD660s showed a more significant performance range depending on the source than the Sundara.
Both perform well for detail retrieval. While these headphones have been available for some time now, they both perform better than similarly priced headphones from the previous generation. For structural definition of images and clarity for the whole frequency range, I give the edge to the HiFiMAN Sundara, and this may simply be the planar tech showing its strengths. But for the midrange specifically (up to around 1khz), I find the HD 660s to be slightly more engaging when using it on high end sources like the SPL Phonitor X and the iFi Pro iDSD DAC. To put it another way, whenever I hear horns or brass sections, I get more textural nuances on the HD 660s, but for everything else, I get more from the Sundara. For more affordable sources like the iFi iDSD Micro BL, I found the Sundara to be superior within that range as well.
Winner: Slight edge to Sundara, depending on source equipment.
Speed & Dynamics
The HD660s performs surprisingly well for speed, once again scaling better on more high-end equipment. But I don't think it's quite on the same level as the Sundara. The planar magnetic driver in the Sundara allows it to have an unmatched 'tightness' and control - especially for bass frequencies. This is where that well-known planar bass performance shows up. Additionally, I find the Sundara to have a bit more slam as well, but this perception could be due to differences in bass levels rather than actual excursive ability. Still the Sundara's punch and impact are surprisingly good - even relative to the rest of the HiFiMAN lineup.
Soundstage & Imaging
For most performance categories you could give an edge to one or the other depending on both preference for transducer type and what equipment you run them with. But for soundstage and imaging, the Sundara is the clear winner. The HD 660s doesn't improve the soundstage from the HD 650 / HD 6XX, so it's a very "in your head" type of experience, especially for the front image. And, while the Sundara isn't particularly spacious either, it's certainly more so than the HD 660s.
Importantly, the image distinction and precision is better on the Sundara as well. I don't find the HD 660s to be lacking here at all - in fact I find it better than most other headphones in this price range (again, more noticeable on the SPL Phonitor X). But this is also a feature of planar magnetic headphones. Instrument separation is often better than similarly priced dynamic driver headphones, and with these two I find that to be the case.
These measurements were taken with the MiniDSP EARS rig, using both the HEQ and HPN compensations. This measurement system is not industry standard and should not be compared with other measurements that are. Note that there is a coupler artifact at 4.5khz that shows up on just about every headphone.
The following shows how these headphones measure relative to the HPN compensation, which is closer to a traditional diffuse field target and doesn't take the Harman bass elevation into consideration.
The following shows how these headphones measure relative to the HEQ compensation, which is based on the Harman target and does add a bass shelf. Headphones that show flat bass extension on this compensation will have more bass than headphones that show flat bass extension on the HPN compensation.
Both the HD 660s and Sundara perform well for their frequency response and tonality. For the most part, everything retains tonal balance throughout the whole frequency range, with a few minor differences that are worth discussing.
The first difference is that the Sundara has better bass extension. This may be surprising to anyone who has read older reviews of the HiFiMAN Sundara where it showed a bit of a bass roll-off above 50hz. But at some point there was a revision to the pads, and this has resulted in improved bass response. The HD 660s does have improved bass response over the HD 6XX, but it still rolls off a bit when compared with the Sundara. We also see a bit more midrange energy on the HD 6XX, and this likely explains the more noticeable character to certain instruments. In general, I can see why people like the HD6__ midrange. It's slightly elevated to give a bit of character, but not overdone in a way that muffles the presentation. The Sundara's midrange is a bit more linear, owing in part to the better bass extension as well.
Moving past the 4.5khz rig artifact (there is no bump there like it shows on the HEQ compensation for both headphones), we see the Sundara has slightly more lower treble energy, and a more gradual elevation to the consonant range at 8.5khz. To my ear, this gives the Sundara more clarity and resolution for piano tones and acoustic guitars. The resonant harmonics that trail the initial tone come through more noticeably. And then above 10khz, the Sundara also has a bit more energy, meaning that the overall 'resolution' for percussive instruments and cymbals hits is also a bit better.
'Resolution' is a difficult term to properly make sense of, and people have different ways of describing this, but to me it's the frequency response related elements of tonal balance that contribute to overall clarity. So if a headphone achieves good tonal balance in its frequency response, it more closely matches the way we'd naturally hear sounds that token those ranges. In this case, for both headphones, the primary hit for cymbal tones isn't out of balance with the splash or sizzle quality that comes afterwards. There's no 5khz peak that causes snare hits to sound congested. Vocals don't sound nasally - it's all very natural. But with that said, the Sundara's treble still has better overall tonal balance and and clarity for the instruments that token the upper frequency ranges - at least to my ear.
Because the HD 660s has excellent detail retrieval capabilities, this is only a difference I really notice when volume matched and swapping back and forth between the headphones. Prospective buyers will be happy with treble resolution for both headphones, they're both airy and open sounding. But comparing side by side, the Sundara is just slightly more resolving for treble, at least to my ear.
Winner: Sundara - barely
The Sennheiser HD 660s is an excellent headphone. Barring a minor concern about lack of stage, there really isn't anything to complain about when it comes to the HD 660s. Its only problem is that the HiFiMAN Sundara performs slightly better, and is less expensive. I think it's important to remember that the Sundara used to cost around the same as the HD 660s, and if they were both the same price, it would be a more fair fight. I think I'd still prefer the Sundara, but just barely, and it would also depend on the source equipment I had to run it. For most people looking to get into this hobby and want a step up from the traditional HD 6XX that everyone has been buying for years, the Sundara is a bigger, yet perhaps less familiar step. Moreover, it might be time for Sennheiser to re-evaluate the HD 660s' pricing and take a good look at the competition - not just from HiFiMAN, but their own Drop HD 6XX is less than half the price and not that far behind.
Winner: HiFiMAN Sundara
Review written by Andrew Park (@Resolve)
Check out the video comparison here: