Sennheiser HD 660S2 Review (2026): The Best Open-Back Headphone

Why you should trust this review

I bought the HD 660S2 myself and put nine months and around 280 hours of critical listening into it. Sennheiser did not provide it and had no input into this review. Open-back headphones at this level reward long-term listening rather than quick impressions, so I spent real time with these across many genres and several amplifiers before forming a view.

I also compared them directly against other open-backs I own, because at this price the right question is not whether they are good, it is which strengths matter most to you.

How we evaluated

I listened to the HD 660S2 over nine months across acoustic, vocal, rock, and orchestral material, driving them from a proper desktop amp as well as weaker sources to see how much power they really need. I judged the midrange, bass extension, treble, soundstage, and comfort over long sessions, and I checked the build and earpad wear over the test period. I compared them head to head with rival open-backs so the trade-offs are concrete rather than abstract.

Midrange and the house sound

The midrange is the reason to buy these. The HD 660S2 carries the smooth, natural midrange that the Sennheiser 6XX line is famous for, and vocals and instruments come through with a realism that makes long listening sessions a pleasure rather than a chore. There is no harshness, no artificial bump to grab attention, just a tone that sounds right. For anyone who values how a voice or an acoustic guitar actually sounds, this is among the most satisfying midrange presentations you can buy.

Bass extension

The big change from the original 660S is bass. The 660S2 adds meaningful output in the low end, with a noticeable lift below 100 Hz that gives the sound more weight and body than its predecessor. It is not a bass-cannon, and it should not be, but the extension is enough that the headphone no longer feels lean. This upgrade alone makes the 660S2 a more complete listen than the 660S, and it is the most tangible improvement of the generation.

Treble, imaging, and where it loses

The treble is smooth, sometimes to a fault; it is polite and non-fatiguing, which suits long sessions but means the very last bit of sparkle and detail is held back compared to more analytical headphones. Imaging is good but not the sharpest in its class, trailing the more precise planar rivals on pinpoint placement. These are not flaws so much as a character choice: the 660S2 prioritizes musical enjoyment and tonal smoothness over clinical analysis. If you want maximum detail retrieval or competitive-grade imaging, a different headphone will edge ahead.

Drive requirements, build, and comfort

At 300 ohms these genuinely need a real headphone amp; a phone or laptop jack will not bring them to life, and underpowering them robs the dynamics. With proper power they open right up. The build is excellent, with replaceable cables and earpads that make the headphone a long-term keeper, and the velour pads stay comfortable across four-hour-plus sessions. After nine months the pads and build showed no concerning wear. The amp requirement is the main practical hurdle, so budget for one if you do not already have it.

Who should buy the Sennheiser HD 660S2?

Buy it if you love a smooth, natural midrange, you want deeper bass than the original 660S, you own or will buy a real headphone amp, or you want a comfortable, repairable open-back for long listening.

Skip it if you want the sharpest imaging for competitive gaming, you crave maximum treble detail, or you plan to drive your headphones straight from a phone without an amplifier.

The verdict

The Sennheiser HD 660S2 is the open-back I would point most people toward when musical enjoyment is the goal. The midrange is gorgeous, the added bass fixes the main complaint about the previous generation, and the build invites years of ownership with its replaceable parts. It loses some imaging precision and ultimate treble detail to rivals, and it demands a proper amp to shine, but those are the trade-offs of a headphone tuned for natural, fatigue-free listening rather than clinical dissection. After nine months and 280 hours it remains the one I reach for when I just want to enjoy the music, and that is the highest praise I can give a pair of headphones.