By aspect — in detail
The headline change and the biggest argument. Everyone agrees it extends deeper than the HD 660S and HD 650 yet still isn't a 'bassy' headphone; they split on whether the improvement is a real, satisfying step up or a marginal one on an open-back that's still soft and rolls off in the sub-bass.
Measured
DIY-Audio-Heaven measures sub-bass rolling off below ~40 Hz (typical of open-backs) and finds the marketed 'doubling of SPL (+6 dB @ 20 Hz)' is 'not met and only is +3 dB' — audible as easier-to-hear low rumble, but 'Bass level itself is not higher.' The resonant frequency drops from ~110 Hz on the HD 660S to ~70 Hz, which is the mechanism behind the extra depth.
⚠ vs. listeners — Both camps are right about the fact — it extends a few dB deeper than its siblings but is still a roll-off-below-40 Hz open-back. The disagreement is whether that gain is perceptible and satisfying (owners: yes) or marginal and still soft (measurement/critics: barely).
Where it splits
A real, worthwhile step up — it finally rumbles low where the HD 650/660S roll off, tight and controlled.47%
“the HD 660S2 bass is in another league compared to the HD 650”
Headphonesty
Still a soft, light open-back — the extra extension is marginal and the 'more bass' pitch is overstated.53%
“There's a lack of tautness and punch which also stifles the low-reaching dynamics of the track.”
What Hi-Fi?
The classic Sennheiser midrange — and the crux of the debate. A measured dip around 3–5 kHz pulls the upper mids back versus the HD 600/650. Most hear that as smooth, easy and non-shouty; a critical camp hears it as scooped and 'muffled,' stripped of clarity, and a real downgrade from the family's best.
Measured
DIY-Audio-Heaven notes a small 2–5 kHz dip that makes the upper mids 'laid-back'; the Headphones.com listener attributes the recession to a '3-5 kHz scoop.' Both agree the upper mids sit lower than the HD 600 (more forward) and HD 650 (lusher).
⚠ vs. listeners — The same measured upper-mid recession is heard as pleasant smoothing (non-shouty, fatigue-free) or as a clarity-robbing scoop — a preference split on one physical feature, not a dispute over the graph.
Where it splits
Smooth, refined, easy vocals — the pulled-back upper mids make it relaxed and non-fatiguing.60%
“Vocals sound super-smooth and ooze refinement.”
What Hi-Fi?
The 3–5 kHz scoop robs clarity — vocals sound recessed and 'muffled,' losing the HD 600/650 magic.40%
“The vocals on this headphone sound distant… but not in a "far out soundstage" way, more in a "muffled and hollow" way.”
Headphones.com (listener review)
Broad agreement: smooth, silky and non-fatiguing, extended well past 20 kHz, with a touch more upper-treble sparkle than the HD 660S. The lone caveat is a small ~6 kHz presence lift that a few treble-sensitive listeners find tiring on bad recordings.
“Treble is smooth and 'silky' with just enough 'air' and detail.”
DIY-Audio-Heaven
“lower-treble peak around 6kHz can be tiring for treble-sensitive people”
Headphonesty
Measured
DIY-Audio-Heaven: a small 6 kHz elevation 'too small to cause sibilance' that gives 'just enough bite,' over a treble that extends past 20 kHz without peaks; versus the HD 660S, the response above 5 kHz is a few dB higher, for 'comparatively more sparkle.'
The core identity, and a rare point of near-consensus: warm-of-neutral, relaxed and laid-back — the recognizable Sennheiser house sound, made a touch darker and easier by the recessed upper mids and the gentle low-end tilt.
“A pleasant and slightly warmish-neutral and laid-back sound that does not 'pop' anywhere”
DIY-Audio-Heaven
“There's definitely warmth, but it's a natural warmth, not lush or thick”
Sara Schweiger, Moon-Audio
Measured
DIY-Audio-Heaven: a 1 dB downward tilt from 100 Hz to 1.5 kHz gives 'a slightly warm-ish tonality' with good bass-to-mids integration; there's no HD 650-style mid-bass hump, so the warmth is a tilt, not a bump.
Soundstage
Contested · 6 srcGenuinely contested. A minority hears a notably spacious, out-of-head stage; the measurement-minded and critical camp hears the same intimate HD6xx head-stage as the HD 600/650 — improved in depth, but not wider.
Measured
DIY-Audio-Heaven reads the head-stage as identical to the other HD6xx models; Headphonesty calls width 'average' for an open-back but notes 'stage depth is also improved.'
Where it splits
Spacious and enveloping — one of the more enjoyable stages at the price.40%
“this is one of the most enjoyable soundstages I've experienced. I would even venture to say that the soundstage beats some of the higher priced planar magnetic models on the market.”
Gabby Bloch, Audio46
The same intimate, in-head HD6xx stage — no wider than the HD 600/650.60%
“It isn't wider nor narrower than HD600/HD650 to me.”
DIY-Audio-Heaven
A quiet strength with broad agreement: precise, stable placement and separation, and the trait several reviewers single out as most improved over the HD 660S.
“Imaging is precise with accurate instrument placement.”
Headphonesty
“The quality of the imaging is just so much more satisfying as a result.”
Marcus, Headfonics
Measured
DIY-Audio-Heaven: 'Instruments are easy to place and do not wander around in the stereo-image,' with an almost exemplary phase response.
Good, not class-leading. A clear technical step up on the HD 660S and resolving for a dynamic driver, but it trails similarly priced planars — and the critical camp finds it merely mediocre, partly a casualty of the recessed upper mids.
“Overall detail and resolution are very good, but the HD 660S2 fall behind some mid-range planar magnetic headphones.”
Headphonesty
“In terms of what some call detail or resolution, it's surprisingly mediocre for a headphone that originally cost $600.”
Headphones.com (listener review)
Improved over the HD 660S in macro- and micro-dynamics, but soft in absolute terms — slam and punch are not its strength, a limit tied to the modest bass and the pulled-back upper mids that flatten perceived contrast.
“Better macro and microdynamics than their predecessors.”
Headphonesty
“Dynamically, HD 660S2 is rather soft even compared to planar magnetic headphones like the Hifiman Edition XS.”
Headphones.com (listener review)
Positive and near-universal: light (~260 g), plush velour pads, fine for hours. The one recurring caveat is a firm out-of-box clamp — measured 'high' at 6 N — that most agree eases with use, and a top-of-head design a few feel is overdue for a suspension strap.
“it's like slipping your feet into a pair of posh (and very comfortable) slippers.”
What Hi-Fi?
“The clamping force, right out the box, is quite high (6N) and seems higher than needed.”
DIY-Audio-Heaven
Measured
DIY-Audio-Heaven: ~226 g measured (Sennheiser rates 260 g), velour pads, clamping force 'high (6N)' that 'most likely will get somewhat lower during usage.'
The familiar HD6xx verdict: plasticky and not luxurious — and the box now feels cheap for the price — but robustly made, comfortable to live with, endlessly repairable with off-the-shelf parts, and bundled with both balanced (4.4 mm) and single-ended (6.35 mm) cables.
“Time-tested build that should last years”
Headphonesty
“It has the same build as the decades old HD6** series.”
DIY-Audio-Heaven
Measured
Designed in Germany, manufactured in Ireland; 38 mm dynamic driver on Sennheiser's SYS platform; replaceable 2-pin cables and off-the-shelf spare parts, as with the rest of the family.
Isolation
Moderate · 2 srcOpen-back by design: essentially no passive isolation and it leaks both ways. Expected for the type, not a flaw — but it rules out noisy commutes and shared rooms.
“There is little isolation from outside noises as this is an open headphone.”
DIY-Audio-Heaven
“Isolation is almost nonexistent due to the open-back design.”
Headphonesty
The launch-day controversy. Panned at its $599 MSRP for costing far more than the cheaper, near-identical HD 6XX/650 — and the HD 58X that measures almost the same for less than half — its case improves markedly at today's discounted street price, which is where most owners feel it earns its keep.
Measured
Launched at $599 (€600); street price has since fallen to roughly $320–400. The Drop x Sennheiser HD 58X measures nearly identically at less than half the price, and the HD 6XX/650 sit well below it.
Where it splits
Overpriced — the far cheaper HD 6XX, HD 650 and HD 58X do most of the same for much less.60%
“There's not a single aspect of the frequency response or my subjective sonic impression that would make me recommend the HD 660S2 over the HD 650; the latter is likely going to be a better headphone for most listeners, and is less expensive.”
Headphones.com (listener review)
Fine value once discounted below MSRP — reasonable in the context of the HD 600/650 family.40%
“If you find the HD 660S2 for sale at a significant discount, they are well worth the purchase. At the MSRP, I have my reservations.”
Headphonesty
