By aspect — in detail
The most-argued aspect. Everyone agrees it reaches deeper into the sub-bass than the open 6-series and leans toward mid-bass warmth over slam; they split on whether that low end is a real, satisfying upgrade or a restrained, fit-sensitive one with little impact. Bass level also visibly shifts with seal and head shape, which is part of why sources disagree.
Measured
Headfonics reads the bass shelf as 'almost on target to 100Hz' and ~8–10 dB above the HD 600 up to 50 Hz, with a dipped 100–300 Hz that removes mid-bass warmth. Biodegraded's rig shows strong low bass but a roll-off below ~70 Hz that 'seems to vary between headphones,' and ASR measured high bass distortion — a trait it has seen in Sennheiser headphones before.
⚠ vs. listeners — Both camps describe the same response: more sub-bass than the open 6-series, but tilted to mid-bass and roll-off-prone below ~70 Hz on some rigs and seals. Whether that reads as a welcome upgrade or a restrained, fit-dependent low end is preference — and, literally, a matter of how well the pads seal on your head.
Where it splits
A real, welcome low end — genuine sub-bass the open 6-series never had, with warmth and control.45%
“the bass is substantially lifted from HD600 series which is very welcome”
Amir, Audio Science Review
Restrained and mid-bass-tilted — light on slam and impact, and the deep bass depends on your seal.55%
“I don’t think this is a headphone I’d recommend to someone who wants a really enjoyable presentation for bass in particular”
Headphones.com
The classic Sennheiser midrange, reworked and a little quirky. An unusual 400–800 Hz lift plus a scooped 1–2 kHz tilt the mids warm and non-fatiguing, which most welcome on a closed-back; a minority hears the same tuning make female vocals and piano sound boxy or 'odd,' and nearly everyone agrees it's a touch less natural and refined than the open HD 600 / 650.
“Slightly warmer mids are welcome on a closed back”
Headphones.com
“the response through the mids makes female vocals and piano sound odd, there being too much emphasis on low to middle mids and not enough on upper-mid harmonics”
Biodegraded, audioreviews.org
Measured
Marcus (Headfonics) measures a 1–2 dB lift from 400–800 Hz and a 1–2 kHz that's 'largely neutral,' with higher-pitched vocals 'more forward but not quite as natural or as smooth as the HD 600 or the HD 650.' Headphones.com attributes the boxy note weight on some instruments to that same center-mid elevation.
Broadly smooth, safe and non-fatiguing, with less lower-treble bite than the HD 600 / 660S2 but a lift up in the air region. The catch is minimal driver damping, which leaves a few narrow resonances that the most treble-sensitive reviewers hear as a touch sizzly or glassy — most listeners find it easy-going.
“There is some welcome elevation post 6k, but less energy through the lower treble compared to the HD 660S2 or the HD 600.”
Marcus, Headfonics
“the treble is just a bit too sizzly and glassy for me on most things”
Headphones.com
Measured
ASR's higher-resolution graph shows a 'deep cancellation at 4000 Hz' that also appears in group delay, and which Amir suspects hurts spatial qualities. Biodegraded reads a depressed ~4–7 kHz next to an elevated ~8–12 kHz — a down-then-up treble he believes is a deliberate choice to widen the perceived stage.
Tonality
Contested · 6 srcThe overall signature is described three different ways, which is itself the story. Most call it neutral-leaning and reasonably balanced — closer to neutral than its price rivals — tilted gently warm and easy; a minority reads the same response as an uneven, asymmetric v- or w-shape with scooped mids and a disconnected treble.
Measured
The underlying curve is a gentle downslope with a sub-bass lift, a 100–300 Hz mid-bass dip, a 400–800 Hz hump, a scooped 1–2 kHz, and a depressed-then-elevated treble. ASR calls target compliance 'very good … something we have not experienced in this price range'; Headfonia hears the net result as v-shaped, Biodegraded as an asymmetric w — the same response read through different ears.
Where it splits
Neutral-leaning and balanced — closer to neutral than anything else at the price, tilted gently warm.70%
“we have a reasonably priced headphone that comes close to delivering neutral sound”
Amir, Audio Science Review
An uneven, asymmetric v/w-shape — scooped mids and a down-then-up treble that can sound odd.30%
“Asymmetric w-shaped FR (dips in upper bass, lower treble)”
Biodegraded, audioreviews.org
Soundstage
Moderate · 9 srcThe headline feature, and a real one: source after source calls the stage unusually wide and open for a closed-back, an intentional product of the angled baffle. The honest caveat, from the two most measurement-minded reviewers, is that it's wide for a closed-back rather than genuinely open, and depth and separation are only adequate.
“they provided a wide soundstage, more like what I would expect from an open-back design”
Home Theater HiFi
“Is it like an open headphone? Absolutely not.”
Headphones.com
Measured
ASR's listening notes are the dissent: 'Spatial aspects were muted,' which Amir ties to the 4 kHz cancellation. Biodegraded, who otherwise calls staging a highlight, still puts it as a clear improvement on the 'narrow and 3-blob character' of the open HD 600-series rather than a truly open presentation.
A consistent strength that pairs with the wide stage: stable, well-organized left-center-right placement that several reviewers single out as a highlight for a closed-back. The one recurring nitpick is that fine instrument separation and edge definition are good rather than pinpoint.
“Imaging is excellent.”
Home Theater HiFi
“Staging and imaging are highlights of these headphones.”
Biodegraded, audioreviews.org
Solid but not a class-leader, and honest reviewers flag that some of the perceived detail is a byproduct of the treble tilt rather than true resolution. It clearly trails the open HD 600 / 650 — the price-class reference for resolution — which even its fans concede.
“Detail/resolution and clarity, particularly from the upper mids and higher in the range, are very good; although some of this perception is likely due to the treble tilt in the frequency response”
Biodegraded, audioreviews.org
“the sense of “detail” here isn’t anything special for its price on my head”
Headphones.com
The most-agreed weakness. Slam, punch and dynamic contrast are soft in absolute terms — a recurring note from mainstream and critical reviewers alike — tied to the mid-bass dip and scooped upper mids. It's an easy, relaxed listen rather than a punchy, exciting one.
“the sense of dynamic contrast and punch on this headphone is among the poorer options I’ve tried, regardless of price”
Headphones.com
“Could do with a greater dose of punch and dynamics”
What Hi-Fi?
Mixed, and fit-dependent. The thick synthetic-leather pads and headband padding are widely praised — some call them the most comfortable in the 6-series — but a firm initial clamp is a near-universal caveat that most agree eases with use, and can bother larger heads or glasses-wearers. Try before you buy.
“Most comfortable pads on a HD6 Series headphones to date”
Marcus, Headfonics
“It clamps rather heavily on my head causing mild discomfort.”
Amir, Audio Science Review
“lots of clamp but rapidly loosening up the more I use them”
r/headphones owner (via RedditRecs)
Measured
Measured weight runs ~309–326 g (about 60 g heavier than the open HD 600 / 650 / 660S2). Biodegraded notes clamping force ~25% higher than the open 6-series, which is why bigger heads may need to stretch the band.
A quiet strength. Steel-reinforced earcup covers and headband sliders make it feel sturdy and 'tank-like,' with easily replaceable pads and cable. The caveats: the frame is still mostly plastic and doesn't look premium, the twist-lock cable connector is awkward, no balanced cable is included, and a few units creak.
“The ear cup housings and headband sliders are made of reinforced steel and look like they will take a beating.”
Home Theater HiFi
“the frame does feel a touch plasticky, but the reinforced metal in the headband and earcup housings make the whole arrangement feel sturdy”
What Hi-Fi?
Isolation
Moderate · 6 srcThe reason to pick it over the open 6-series, and it delivers: genuinely good passive isolation and low leakage for a sealed dynamic. The honest limit is that it's passive, not ANC — it hushes keyboards and domestic chatter but loud noise still gets through, so it's a home/office headphone, not a commuter.
“If you stand next to somebody using the HD 620S, you hear absolutely nothing, zilch, zero.”
Jürgen Kraus, audioreviews.org
“loud sounds still push through – you won't fare too well with commutes, offices or particularly rowdy housemates”
Henry St Leger, TechRadar
The defining argument. As a closed-back at ~$350 it's widely called the best of its kind and a legitimate 6-series member — even its sharpest critic says it earns the badge. But because it sits at the same price as the open HD 600 / HD 6XX and trails them in resolution and refinement, the measurement-minded camp treats it as a sidegrade: worth it if you need isolation, skip it if you don't.
Measured
Launched at $349.95 (ASR's unit shipped at $300), placing it between the HD 569 and the open HD 600 / HD 6XX it's most compared to. Reviewers repeatedly frame it as 'a closed HD 600 with added bass' — the crux of the sidegrade debate.
Where it splits
Class-leading — arguably the best passive closed-back around $350 and a real 6-series headphone.68%
“I think HD 620S actually does earn its spot in the 6 series. Certainly more than any of the 660S series headphones ever did.”
Headphones.com
Only if you need a closed-back — the cheaper open HD 6XX / 600 / 650 out-resolve it, so otherwise it's a sidegrade.32%
“So if the insulation of a closed-back is not a must-have, I’d have no hesitation in choosing the HD 600 or 650/6XX over the HD 620S.”
Biodegraded, audioreviews.org
