Sennheiser HD 660S2

HiFi, OpenBack
Sennheiser HD 660S2 — The HD600 Gets a Younger Sibling

As a devoted HD600 owner since 1997, any new Sennheiser in the 6-series range gets my immediate attention. The HD 660S2 is Sennheiser’s latest revision, and the question every HD600/650 owner asks is simple: is this the upgrade I’ve been waiting for, or another sidegrade?

Build & Design

If you’ve ever held an HD600, the 660S2 will feel instantly familiar. The same general form factor, the same reassuring clamp, the same utilitarian German design philosophy. The updates are evolutionary: the headband padding is improved, the finish is slightly more refined, and the cable terminates in a 4.4mm balanced connector (with a 6.3mm adapter included).

At 260g, it’s marginally lighter than the HD600, and comfort remains exemplary. These are headphones designed for eight-hour mixing sessions, and they deliver. The build quality is solid without being extravagant — Sennheiser’s approach has always been “let the sound do the talking.”

Sound

Bass

Here’s the biggest departure from the HD600. The 660S2 has noticeably more bass presence — not in a bloated or excessive way, but there’s genuine sub-bass extension that the HD600 simply doesn’t offer. Bass guitar has more weight, electronic music has more foundation, and kick drums have more body. Crinacle noted the improved bass response as the most significant tuning change, and I’d agree — it transforms the listening experience on bass-heavy genres.

Mids

Classic Sennheiser. The 660S2 retains that beautiful midrange clarity that made the 6-series legendary. Vocals are intimate, natural, and emotionally engaging. There’s perhaps a fraction less upper-mid presence compared to the HD600, which makes the 660S2 slightly more relaxed and less forward. Some will prefer this; purists may miss the HD600’s raw midrange honesty.

Treble

Smoother than the HD600, with less of the 6kHz peak that can occasionally become noticeable on bright recordings. The 660S2’s treble is refined and extended without harshness. Detail retrieval is excellent — you hear everything, but nothing is thrown in your face. It’s a mature presentation that serves both music enjoyment and professional monitoring.

Soundstage & Imaging

Similar to the HD600 — intimate rather than expansive, with precise imaging within that space. Head-related transfer is natural, and centre imaging is rock-solid. You won’t get the vast soundstage of an Arya, but for vocal-focused music and small ensemble work, the 660S2’s presentation is deeply convincing.

Dynamics

Improved over the HD600, particularly in the bass region where the added extension gives more dynamic range to work with. The 300-ohm impedance means it benefits from proper amplification — through my Sparkos Gemini it’s dynamic and engaging; through a phone dongle, noticeably less so.

Comparisons

Against my beloved HD600, the 660S2 is a genuine upgrade in bass and treble refinement, with a very slight trade-off in raw midrange forwardness. If I could only keep one, it would be a genuinely agonising choice. The HD600’s mids remain magical; the 660S2’s bass is transformative. Resolve rated the 660S2 as one of the best mid-range Sennheisers, and I understand why.

Against the Meze Poet, the 660S2 is more neutral, more precise, and less warm. The Poet flatters; the 660S2 informs. Different tools, both excellent.

Against the Focal Clear MG, the 660S2 is less dynamic and less resolving, but considerably more natural in the mids and far less demanding of source quality. The Clear MG needs careful system matching; the 660S2 sounds good with everything.

Verdict

Pros

  • Improved bass over HD600 without losing midrange magic
  • Refined, non-fatiguing treble
  • Outstanding comfort for all-day wear
  • Balanced 4.4mm connector out of the box

Cons

  • Soundstage remains intimate
  • Needs proper amplification at 300 ohms
  • Evolutionary rather than revolutionary over HD600
  • Premium price for the incremental upgrade

Ratings:

  • Build & Design: 7.5 / 10
  • Sound: 8.5 / 10
  • Comfort: 9.5 / 10
  • Value: 7.5 / 10

If you love the HD600 but wish it had more bass, the 660S2 is the answer you’ve been waiting three decades for.