The latest update to Sennheiser's legendary 600-series lineup revises the voice coil to match the 300-ohm impedance of the classic HD 600 and HD 650, delivering cleaner impulse response and stronger magnetic damping. A more compliant driver surround drops the resonant frequency to 70Hz, and reworked airflow around the transducer roughly doubles sound pressure at the lowest frequencies compared to the original HD 660S.

Open-back circumaural design keeps long listening sessions physically comfortable, with breathable ear cushions and a light, ergonomic headband. Built in Ireland with German engineering, it ships with both balanced (4.4mm) and unbalanced (6.35mm) cables plus a 3.5mm adapter.