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. It demands a capable amplifier but rewards with detail retrieval and imaging that rival headphones costing twice as much.

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 weight comfortably across the crown, though clamping force is slightly higher than the HD 600, requiring a break-in period of about 20 hours. Ear pads are user-replaceable via a twist-lock mechanism. The included cables - both a 6.35mm single-ended and a 4.4mm balanced cable - are a substantial improvement over prior generations with tangle-resistant braided sheathing.

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 dedicated amplifier - paired with a Schiit Magni+ or equivalent, 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 of the original HD 660S. Imaging is precise across the soundstage, with instrument separation that rivals headphones at much higher price points, though stage width is narrower than the HD 800 S by a clear margin.

Features

|The 150mm dynamic driver uses a 38mm magnet assembly that Sennheiser claims reduces distortion by 30% compared to the HD 660S. The frequency response extends to 41.5 kHz, and the dual cable system includes both single-ended and balanced connectivity. The open-back design provides a natural, speaker-like presentation but leaks sound significantly, making them unsuitable for shared spaces or commuting. The included carrying case is basic but functional.

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 that made the series legendary. It demands good upstream gear but rewards with some of the best detail retrieval available under $500. The open-back design and 300-ohm impedance make it a serious tool for dedicated audiophiles.

Sources

• Sennheiser HD 660S2 Product Page
• Sennheiser HD 660S2 Specifications
• SoundStage! Solo - Sennheiser HD 660S2 Review