Overview

The Sennheiser HD 660S2 refines the legendary HD 600 series with improved bass extension, a redesigned magnet system, and a more nuanced treble response. At $499 it sits between the HD 600 and HD 800 S, targeting audiophiles who want reference tuning with a touch more warmth and sub-bass presence.

Design & Build

The HD 660S2 retains the classic 600-series open-back aesthetic with velour earpads, a matte black plastic frame, and the iconic oval ear cups that have barely changed in two decades. The headband uses the same split-metal suspension design that distributes 260 grams comfortably across the crown, though the clamping force is slightly higher than the HD 600, requiring a break-in period of about 20 hours for optimal comfort. Ear pads are user-replaceable via a twist-lock mechanism, and the included cables are a substantial improvement over prior generations with tangle-resistant braided sheathing. The entire assembly feels utilitarian rather than luxurious, but build quality is solid and serviceable with widely available replacement parts.

Performance

Bass extension now reaches convincingly to 30 Hz with a 6 dB elevation over the HD 600, giving kick drums and synth bass real weight without bleeding into the mids. The 300-ohm impedance demands a capable amplifier; paired with a Schiit Magni+ the HD 660S2 delivers tight, controlled low-end and a midrange that preserves the vocal intimacy the series is famous for. Treble is airier than the HD 600 but avoids the sibilance peak that some listeners found fatiguing in the HD 660S original. Imaging is precise across the soundstage, with instrument separation that rivals headphones costing twice as much, though the stage width is narrower than the HD 800 S by a noticeable margin.

Features

The dual cable system includes both a 6.35mm single-ended cable and a 4.4mm balanced cable, a welcome addition for users of modern DAC/amp combos with balanced outputs. The 150mm dynamic driver uses a 38mm magnet assembly that Sennheiser claims reduces distortion by 30 percent compared to the HD 660S. Frequency response extends to 41.5 kHz, though real-world audibility above 20 kHz is negligible. The open-back design provides a natural, speaker-like presentation but leaks sound significantly, making these unsuitable for shared spaces or commuting.

Pros

• Reference-level midrange with improved bass extension over HD 600
• Excellent imaging and instrument separation for the price
• Replaceable earpads and cables for long-term serviceability
• Balanced cable included in the box

Cons

• Requires a dedicated headphone amplifier for full performance
• Clamping force is high out of the box
• Open-back design leaks sound and offers zero isolation
• Plastic build feels less premium than competitors at this price

Verdict

The Sennheiser HD 660S2 is a thoughtful evolution of the HD 600 formula, adding bass presence and treble refinement without sacrificing the midrange magic. It demands good upstream gear but rewards with some of the best detail retrieval available under $500.

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