Disclosure: I’d like to thank Sennheiser for sending me the HD 660S2 for this review. All thoughts and opinions are my own, and Sennheiser had no say or input for the contents of this review.

The Sennheiser HD 660S2 represents the latest evolution in Sennheiser’s acclaimed 600-series open-back lineup, bringing notable improvements to the already excellent HD 660S. These $599 USD open-back headphones bring meaningful upgrades through its new 38mm transducer and ultra-light aluminum voice coil, delivering a natural sound signature

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• Sennheiser HD 660S2
• 1.8m cable with 6.35mm connector
• 1.8m balanced cable with 4.4mm Pentaconn
• 6.35mm to 3.5mm adapter cable
• Soft carrying pouch
• User manual

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The jet-black finish with bronze accents gives the HD 660S2 a sophisticated yet understated look. The build quality impresses with attention to practical details. Each earcup has its own cable connection in a dual-entry system, adding durability where it matters. Sennheiser opted for a lightweight plastic frame paired with a spring steel headband, while the earcups feature plush microfiber velour padding.

The open-back design uses a protective mesh grille that allows the drivers to breathe freely. A nice touch is the three raised dots near the left earcup’s headband for easy orientation by feel. The yokes provide a good range of motion side-to-side, and the earcups rotate about 15 degrees front-to-back.

Despite weighing just 260 grams, these headphones initially clamp quite firmly. The pressure distributes well across the head though, avoiding any sharp pressure points. After some use and light stretching of the headband, the clamping pressure lessens and becomes more tolerable. The velour earpads feel soft and comfortable against the skin. The open-back design adds comfort by letting your ears breathe, though this obviously means there is no isolation from outside noise.

After proper adjustment and lessening the clamping force, I’ve worn these comfortably for 6-hour listening sessions without issue.

The chassis hasn’t shown any signs of wear despite regular use. What really impresses me is the modular design – everything from earpads to headband padding can be replaced when needed. The detachable cables only add to this, though Sennheiser uses their proprietary 2-pin connection. With proper care, these could easily become generational audio gear.

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The HD 660S2 introduces several meaningful upgrades over its predecessor. The new 38mm transducer paired with an ultra-light aluminum voice coil delivers enhanced sub-bass response while maintaining the natural, relaxed sound signature the 600 series is known for.

The HD 660S2 presents a warm, natural sound profile with excellent linearity between the lower and upper bass regions. This adds a sense of authority to the lower frequencies without muddying the overall presentation. The overall sound stays remarkably clean even when pushing these headphones hard. I really enjoyed Rock, Metal, Electronic, and Acoustic genres with the HD 660S2, but Jazz and classical music also sounds great too.

Bass

The bass response here really surprised me. The redesigned transducer doubles the sound pressure in the lowest octaves, giving about 5-6dB more presence in the sub-bass region. This means kick drums and bass guitars have proper weight and impact without bleeding into the midrange. The bass stays controlled yet musical, which is exactly what you want from a reference-class headphone. Those looking for deep rumble and slam will be disappointed though, but given these lean towards reference it makes sense. I’m not a bass head but I do like good bass presence, and the 660S2 definitely provides when needed.

Midrange

The midrange might be where these headphones truly excel. They have a presentation that sounds incredibly natural with both vocals and instruments. Unlike many headphones that push the midrange too far forward, the HD 660S2 maintains excellent balance and layering. Acoustic instruments in particular showcase the headphone’s ability to reproduce subtle textures and harmonics. Vocals here are perfect for me as they are slightly forward but not overly emphasized. Male vocals retain their tone and are accurately conveyed, which is evident when listening to something like Borracho by Mark Lanegan, or Lucky by Radiohead. Female vocals also sound spot on. It’s rare for a headphone to do both female and male vocals well.

Treble

The treble has excellent extension, but without any harsh peaks that could cause fatigue. Cymbals and other high-frequency instruments maintain their natural character while avoiding any piercing qualities. There’s just enough elevation around 6kHz to keep things detailed and engaging without becoming aggressive. The result is a treble presentation you can enjoy for hours without fatigue and is suitable for both critical and causal listening.

Soundstage and Imaging

The soundstage is quite open, with excellent width and height, and good depth. It’s not the most spacious I’ve heard in an open-back, but it does the job quite nicely. If you listen to spatial audio these will suffice nicely. Imaging performance is very good, with accurate instrument placement and layering.

Good technicalities usually mean good gaming performance, and the 660S2 is no exception here. They perform very well for single-player open-world games like Skyrim and Cyberpunk 2077, but I found them lacking for competitive games like Call of Duty and PUBG. This isn’t necessarily a slight against the HD 660S2 specifically, but open-back headphones in general. I like to be focused and immersed when playing competitive games, and open-back headphones let in too much noise from my mouse and keyboard, PC fans, and even environmental noise from the busy streets outside my apartment. If those are non-issues for you then the HD 660S2 will work just fine for gaming.

Listening Experience

Since the HD 600S2 are open-back headphones, I think it can be safely said that these headphones simply don’t work for commuting or office use. Outside noise flows freely through, making it impossible to enjoy music over train rumbles, office chatter, or traffic. Sound leakage is equally substantial as others will hear music emanating from the headphones. If you need good-sounding headphones for shared spaces or public transport, I’d recommend looking at the Sennheiser HD 620S, my current closed-back favorite.

The HD 660S2 truly comes alive in quiet home environments. The open-back design creates a natural, spacious soundstage that makes you forget you’re wearing headphones. Testing across genres revealed impressive versatility – rock, blues, jazz, and folk all sound remarkably balanced and engaging. Vocal performances stand out with their clarity and presence.

Those velour earpads proved their worth during long listening sessions. Even after 5 or 6 straight hours, comfort never became an issue. The headphones scale nicely with different sources too – from basic portable devices to high-end desktop setups. Pairing them with quality solid-state amplifiers brings out additional speed and impact, though they don’t demand massive power to

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The Sennheiser HD 660S2 is an excellent addition to the 600-series. The enhanced sub-bass response and natural midrange presentation particularly stand out, making these $599 USD headphones a worthy investment for the right listener.

Sound quality defines the HD 660S2’s character. These headphones truly excel in quiet home environments, where the open-back design creates an immersive experience that makes you forget you’re wearing headphones at all (once the clamping force is lessened, that is). If you’re looking for fatigue-free listening suitable for most music genres, and are interested in both critical and casual listening, I can definitely recommend the HD 660S2 to you.

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