Review: Sennheiser HD 660S2

The word audiophile is as loaded as they come. Members of this unofficial club span the gamut from those who appreciate the value of a good deal on critical listening gear to those ceaselessly chasing the sonic dragon in pursuit of perfection. For the heaviest addicts, concepts like time and money lose all meaning. There is no price too high to pay for what they think will provide improved performance.

Acclaimed European audio brand Sennheiser has deftly walked the line between those extremes for decades. The company’s HD 600 series of wired, open-back headphones are classics in listening circles both casual and critical. At the lower tier of the scale is the HD 6XX, an awesome joint venture between Sennheiser and Drop based on Sennheiser’s classic HD 650. They are incredible reference cans for just over $200.

Jump up a few steps in the series and you’ll find the new HD 660S2, at $600. An update of the HD 660S, the 660S2 add deeper bass extension along with a few other modifications. True to their promise, they sound incredible. But are they worth nearly three times as much as their cheaper sibling? The answer may depend on where you draw your audiophile lines.

Luckily I was excited about the HD 660S2’s arrival, because the initial unboxing can take some air out of your sails. The headphones arrive in a decidedly ordinary box for this price bracket, nestled within a thin cardboard cradle. There’s no hard case, only a satiny drawstring bag: It doesn’t exactly scream luxury.

Things begin to look up as you pull out the other accessories, including a quarter-inch to 3.5-mm adapter, a 6-foot unbalanced quarter-inch cable that’s already attached, and a second balanced cable with a 4.4-mm connector. If you’re wondering about the difference, balanced cables are designed to reduce electrical interference for a clearer signal, though I used the unbalanced cable for my review and had no issues (most folks won’t).

Another bit of technical jargon comes with the HD 660S2’s higher impedance level than their predecessors (300 ohms vs 150 ohms). Impedance is one of those audiophile debates that can turn into a full-blown argument. For headphones, higher impedance generally means more tightly wound voice coils for the drivers, which can reduce distortion, but also benefits from a more powerful source device than a standard phone or laptop can offer.

That said, you can use the HD 660S2 with a laptop (I tried), and they’re rated at the same impedance as the cheaper HD 6XX. The best reason for using a solid headphone amp (besides the extra power) with such headphones is quality components like a good DAC (digital to analog converter), which provides a better signal chain from your audio source to improve the overall performance. Using an amp with a high-impedance setting also provides louder sound with the knob turned lower.

For most of my listening, I connected the 660S2 to a Klipsch Heritage headphone amp using the high impedance setting with a mix of hi-res and compressed audio tracks. I also used an Astro & Kern A&Futura SE100 portable audio player and my HP laptop to switch things up.

You’ll recognize any member of Sennheiser’s HD 600 series from a mile away thanks to their gargantuan, open-back earcups and honeycomb-shaped dynamic drivers. It’s an iconic look you can’t hide—not that you’ll likely be taking these cans on the bus, since the open-backed design leaks sound to the outside world.

Set beneath glittering metal screens, the open-air drivers are much more about function than form, aimed to create clear airflow and extend the soundstage so your experience is more akin to listening to good speakers than stuffy headphones. The 660S2 bring their own aesthetic tweaks to the core design, including a matte-black frame and copper-colored logos along the large band and earcups in place of silver or blue.

On the headphones’ interior, you’ll find healthy heaps of padding up top and plenty more around the earcups, covered in fuzzy velour. The cups swivel both horizontally and vertically to properly cradle your ears, jaw, and temples.

All of this is aimed at luxurious comfort, but the HD 660S2’s aggressive clamping force on my medium-size head puts a damper on the experience for me, while also giving me the worst case of Jim Halpert hair of my life. I don’t recall this much force in the HD 6XX, and it appears to be a signature of the 660 models. The fit is likely to soften over time, and they seem to be wearing in a little, but after weeks the clamping is still inordinately tight each time I strap the headphones on. These are better for smaller-headed listeners.

Fit aside, if you haven’t listened to a pair of quality reference headphones before, it’s one of life’s most satisfying musical experiences you can have. The term “reference-level” essentially means the headphones reveal the true nature of audio as it was recorded, as best they can.

The shades are pulled away, the foundation is fully unearthed, and instruments that were previously masked in obscurity suddenly glitter in the light of day. Songs you’ve listened to for years can sound fresh and new, because cheaper headphones often come with cheaper drivers that are tuned to exaggerate low and high end more than you might hear otherwise. A camera comparison is apt here: Think of a camera with an Instagram filter that makes things look cool, then think of one with a gorgeously transparent Leica lens.

They’re all aiming for clarity, but much like camera makers, each high-end headphone brand puts its own flavor on the experience. Fans love Sennheiser’s HD 600 series not only for their remarkable definition and dynamics, but also for the subtle way in which they express those revelations. Unlike some headphones that can sound more “clinical” or sharp and cold, there’s a smoother grain to the midrange and treble, while still allowing for full distillation of all frequencies. (The claimed total frequency response is 10 Hz to 41.5 kHz.)

The 660S2 build on that sound with an extra drop of amber in the sound, for even more saturated coloring that’s most pronounced in the low end. The bass may be the biggest reason to consider these over more affordable models in the series. Dropping the needle on hip-hop and electronic tunes unleashes an added rush of foundational power, and it can be a lot of fun to listen to.

The main groove in Biggie’s “Hypnotize” is a laser stick of thump with exceptional balance. It’s a funkier sound than the boom you’ll get from cheap bass bombers, almost rewriting the overall feel with a jazzier flavor. Similarly, Too Short’s “Money in the Ghetto” has a sweet, push-button punch to the main kick that’s expertly articulated. Those aren’t my usual go-to tracks for these kinds of headphones, but they helped to set the stage and sounded great doing it.

The Chemical Brothers’ “Go” is an even more intriguing test, again showcasing the 660S2’s bass skills while also highlighting their lightning-fast transient response. The song’s kick drum is carved with a deep texture of echoed reverb that expands like a bubble at the center image. The excellent instrumental spacing lets you roam freely through the dimensional soundstage, from the synth that sweeps through the stereo image ahead of the chorus to the build-up at the instrumental breakdown that feels like an imminent rocket launch.

Even the kick drum in Maxwell’s Silver Hammer has some extra thump to it, providing a more raucous, yet still refined ride than you’ll get from others in the HD 600 series or beyond. I prefer the 6XX’s more neutral take for such tracks, but it wasn’t a detriment to the experience.

Elsewhere, you’re getting the same kind of lyrical clarity and refinement that first endeared me to the HD 6XX. Reverb tails echo into oblivion; piano tracks let you estimate both the room size and mic placement—the nerd in me was asking, “Was that an open-lid close-up of a grand summed down with the room mic?” (I think it was). The distorted guitar in ELO’s is so textural and visceral, it gives me the cold fibrous crunch of biting into a popsicle.

Even podcasts are more richly portrayed, letting you hear not only how close the voices are to the mic beat by beat, but deeper details like whether Sean Hayes had dairy ahead of the Smartless Jon Favreau interview (he definitely did).

It’s all, in a word, glorious. But to my ears, the HD 660S2 aren’t so much an improvement over other headphones in the series as a slightly different take. Fabulous headphones all, I’m most interested in the best value. As I write this, you can get the original 660S at $320, not to mention the shockingly affordable 6XX at $219. What’s more, I found the 6XX to be the comfier of the two I’ve tried, which is a major consideration for extended listening sessions.

If you’re excited about the extra bass, the richer saturation, and even the tighter flex on your noggin, there’s audiophile wonder to be had in the HD 660S2. Whether that’s worth the extra coin is up to you and your wallet.