Sennheiser HD 620S vs Sennheiser HD 660S2

Sennheiser HD 620S

The HD 620S is a closed-back headphone built on a 42mm dynamic driver, listed at $349.95, rated 150 ohms and 110 dB/Vrms, and easy enough to drive that reviewers report full volume from a phone, an Apple dongle or a game controller. Three points are settled across the coverage: passive isolation is unusually strong for a sealed passive design, out-of-box clamping force is high and eases with wear, and the bass you actually hear depends heavily on seal. Everything else splits. One camp hears a near-neutral, HD 600 adjacent tuning with clean mids and a wide, natural stage for a closed back. The other measures and hears an asymmetric W-shaped response: strong sub-bass, a trough near 200 Hz, a 400-800 Hz hump that reads as boxy, a lower-treble recess and a mid-treble lift that leaves the highs sounding disconnected. A measurement-led review scored it 6.5 out of 10 with a 2.9 predicted-quality mean and called the price hard to justify, while a measurement forum still placed it on its recommended list. Detail and dynamics divide the same way.

Sennheiser HD 660S2

The HD 660S2 is a driver-level revision of the HD 660S rather than a new headphone. The voice coil was reworked and impedance moves to 300 ohms, matching the HD 600 and HD 650. The low end is the headline change: sub-bass extends further and the mid-bass carries more body than the predecessor, with the missing fundamental in lower-register instruments described as finally present. The tuning still stays measured rather than bass-forward, and it is explicitly not aimed at bass-heads. Tonally it sits between the HD 600 and HD 650 with a lean toward the HD 650, cleaner through the mids than the HD 650 and less forward than the HD 600. Treble is where reviewers split: one account hears considerably more bite, sizzle and crispness than the original without harshness, while another frames the profile as relaxed and deliberately not treble-emphasized. Staging is reported as taller and deeper with more precision than the predecessor, though the presentation is still called intimate rather than expansive. A dip through roughly 3-5 kHz is flagged for dimming detail on some tracks, with a few dB of parametric EQ offered as the remedy. The chassis, velour pads and headband carry over unchanged, and the packaging is judged a downgrade. Value is the real fault line: at a street price near $500 one verdict reads it as a clear technical step up on the HD 660S, while another concludes it does not differentiate itself from cheaper models in the same family and tells existing HD 660S and HD 600 owners to save their money.