Sennheiser HD 660S2 Two Years Later: The Reference Can That Grew on Me

When Sennheiser announced the HD 660S2 in early 2023, the audiophile community was caught between nostalgia and skepticism. The original HD 600 and HD 650 had spent over two decades as the undisputed kings of mid-tier dynamic open-backs. The first HD 660S had tried to modernize them with a lower 150-ohm impedance and sharper transient attack, but alienated purists who missed the lush midrange warmth of the 650. Now, two full years into daily use of the HD 660S2, I can say with confidence: Sennheiser finally got the formula right.

Transducer Engineering: Rebuilding the Low End

The chief complaint against the classic 600-series chassis was always sub-bass roll-off. Below 80 Hz, dynamic open-backs traditionally drop like a stone, leaving electronic kicks and cinematic sub-bass feeling hollow. With the S2, Sennheiser lowered the resonant frequency of the 38mm transducer from 110 Hz down to 70 Hz. They also revised the acoustic airflow around the voice coil and returned to a 300-ohm impedance, matching the coil specs of the legendary HD 600 and HD 650.

×

In listening sessions, the difference is not subtle. The sub-bass now extends cleanly down to 30 Hz with genuine visceral impact. When an acoustic kick drum hits on a jazz track, or when deep ambient drones swell in an electronic mix, the S2 conveys physical weight and texture without bleeding into the lower midrange. It does not turn the headphone into a bass-cannon—this is still a strictly neutral reference tuning—but it eliminates the sensation that the bottom octave has been amputated.

Midrange Purity and Soundstage Realities

Where the HD 660S2 justifies its existence every single day is vocal presentation. Vocals sit dead center with an intimate, lifelike realism that few planar magnetic headphones can match at twice the price. String instruments possess natural harmonic resonance, acoustic guitars have tactile fingerpick snap, and there is zero fatigue even after six hours of continuous listening. The dreaded 'Sennheiser veil' of the older 650 is completely gone, replaced by a smooth, detailed top-end that never strays into harsh sibilance.

The tradeoff is soundstage width. If you are coming from an expansive planar open-back like the Hifiman Edition XS or an HD 800S, the 660S2 will feel distinctly intimate. Instruments sound like they are situated three feet from your ears rather than in a sprawling concert hall. For critical monitoring, podcast editing, and long focused writing sessions, however, this intimacy is an asset: you are inside the music, not struggling to track distant instruments across an artificially wide panorama.

Comfort, Build & Amplification Requirements

At 260 grams, the HD 660S2 is a featherweight compared to 450-gram planar magnetic competitors. The velour pads breathe naturally during humid summer afternoons, and the plastic chassis—while less luxurious than milled aluminum—is built like a tank. Every single element, from the headband cushion to the cables and earcups, is modular and user-replaceable with standard Sennheiser parts.

Amplification is not optional if you want the S2 to shine. While modern Apple silicon MacBooks will detect the 300-ohm load and boost voltage to an acceptable volume, pairing the S2 with a dedicated desktop DAC/amp (such as a Schiit Magni/Modi stack, a FiiO K7, or a warm OTL tube amp) tightens the bass control noticeably and opens up dynamic contrast. Sennheiser includes both a standard 6.35mm unbalanced cable and a 4.4mm balanced Pentaconn cable in the box.