Sennheiser HD660S2

Sennheiser carefully experimented on driver backwave dampening in the magnet hole of HD660S2. By removing damping material that had been characteristic to HD600 and HD650's design – and bass shortfall – Sennheiser were able to lower the resonant frequency of HD660S2's driver from 110Hz to 70Hz.

Cutting the air resistance as HD660S2's driver moved backwards out of phase allowed greater pistonic travel in a surround that could work more effectively producing the long, slow waveforms of low frequencies, without needing to be more compliant. 

Sealing and optimising airflow around Sennheiser's famous Duofoil transducer also channels different frequencies more efficiently towards the ear and preserves a shallower dB/octave roll-off in HD660S2's bass – directly also responsible for its greater frequency and phase performance down low.

By working for, not against, Sennheiser's customary 38mm dynamic driver that's able to climb in frequency without cone or modal break-up owing to its humble proportions, modernised cup acoustics allow HD660S2 greater pistonic excursion to produce bass waveforms. The maximum potential is milked of this driver.

It convincingly pulls off the best low-frequency performance in a HD6XX headphone yet. HD660S2 is back to being a low-distortion affair too, with a return to a 300ohm voice coil kept thin and light – 10 per cent lighter than its predecessor. Inertia is low and Sennheiser's driver reacts instantly to the smallest voltage impulses.

Indeed, it's a 300ohm load that's simultaneously easy to drive for any partnering amplifier, such that a tube amplifier mates comfortably with high damping factor and impedance matching. 

HD660S2 comes terminated in balanced 4.4mm also meaning many modern bridge-tied load outputs on music players won't grind their gears pushing into inordinately low impedances, small power supplies and output stages producing less in the way of distortion and noise as a result.

This Sennheiser will make for a happy result running from iBasso Audio's DC04 Pro and DX170. But even high output impedance, imperfect voltage sources from FiiO and Shanling will not be tested untowardly either – music players from the latter both having enough mettle to push HD660S2 very satisfactorily. 

More bass, more ways to enjoy it make for more reasons to get acquainted with Sennheiser's latest child – and urgency to own it. Also view Sennheiser's other legends here, or our other headphones here.