The Sennheiser HD 660S2 are the next generation of the Sennheiser HD 660 S and are open-back audiophile headphones. While they look and feel similar to their predecessor, the manufacturer has re-tuned their sound to deliver a bit more thump, punch, and warmth. Their treble range has also received some sculpting, though this change may be a hit or a miss, depending on if you've previously enjoyed the more balanced sound from other Sennheiser headphones like the Sennheiser HD 650. In addition, their impedance has risen from 150 ohms to 300 ohms (with a sensitivity of 104 dB/SPL), so you'll need a powerful amp to drive them effectively.
Our Verdict
The Sennheiser HD 660S2 are inadequate for sports and fitness. Despite their decently comfortable fit and flat, warm sound, these are designed for critical listening rather than exercise companions. Their bulky fit and cabled design will catch on equipment, and they're not particularly stable. Plus, they need a headphone amp to power them, and they'll leak your music into your environment.
- Consistent audio delivery.
- Well-built and decently comfortable fit.
- Aren't designed to block out background noise.
- Leak a lot of audio.
- Can move around or fall off with large head movements.
- Bulky and wired design can get easily snagged on something.
The Sennheiser HD 660S2 are poor for travel use. They're large headphones that don't fold down smaller and require an amp to push them. As open-backs, they neither block out environmental noise nor do they keep your audio private. Otherwise, their flat and slightly warm sound profile delivers clear audio, and they feel decently comfortable, but they're impractical for travelling.
- Consistent audio delivery.
- Well-built and decently comfortable fit.
- Aren't designed to block out background noise.
- Leak a lot of audio.
- Bulky and wired design can get easily snagged on something.
The Sennheiser HD 660S2 are poor for office work. Due to their open-back enclosures, they can't isolate the noises in your workspace, and they also let a lot of your audio leak into the space and bother colleagues. Since they don't have a microphone, you'll need to use a separate one for calls and online meetings. That said, if your office has four walls and a door, the lack of isolation might not be a problem. These feel decently comfortable and produce a flat and clear sound profile, which you might like.
- Consistent audio delivery.
- Well-built and decently comfortable fit.
- Aren't designed to block out background noise.
- Leak a lot of audio.
- No mic.
The Sennheiser HD 660S2 are wired headphones and can't connect wirelessly.
The Sennheiser HD 660S2 are passable for wired gaming. They're well-made and decently comfortable. As open-backs, they don't output a heavy low bass, but sounds such as footsteps, dialogue, and soundtracks sound clear and balanced. With that said, our unit exhibits stereo mismatch in the bass, and the PRTF doesn't create a very immersive sound like an angled reference monitor; these negatively impact the stereo field and localizability of sound sources in-game. The headphones also don't include a microphone, which is fine for solo games, but you'll need to supply your own for chatting with others.
- Consistent audio delivery.
- Well-built and decently comfortable fit.
- Can move around or fall off with large head movements.
- No mic.
The Sennheiser HD 660S2 have satisfactory audio reproduction accuracy. These over-ears deliver a smooth and flat sound signature without many frequencies jumping out or getting buried in the mix. There's a good amount of high-bass boom, but lower bass frequencies lack weight (like most open-backs). Mids sound natural, while the treble is de-emphasized compared to our target and sounds warm. And, these headphones demonstrate remarkably low harmonic distortion. That said, our unit exhibits stereo mismatch, especially in the bass range, which skews the stereo image. These also don't have a tight group delay in the bass; the lack of heavy bass in the tuning offsets the looseness in the bass a bit, though. The headphones also don't interact with your outer ear like an angled reference speaker either.
The Sennheiser HD 660S2 exhibit awful noise isolation, as expected from open-back headphones. Your environment's ambient sounds will reach your ears and compete with whatever audio you're listening to. They also leak a lot of audio into your space, which is very audible to people nearby.
The Sennheiser HD 660S2 don't have a microphone.
The Sennheiser HD 660S2 have good frequency response consistency. These headphones sound more or less alike between listening sessions, and while there are minor variations for different people in how they sound, it's overall quite consistent.
Performance Usages
Changelog
- Updated Mar 24, 2026: This review has been updated to Test Bench 2.1, which removes the Group Delay test, and rebalances scoring for the Harmonic Distortion test and Audio Reproduction Accuracy performance usage. For more details, consult our full changelog.
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Updated Jul 17, 2025:
This review has been updated to Test Bench 2.0, which adds the following tests: Stereo Mismatch, Group Delay, Cumulative Spectral Decay, PRTF, Harmonic Distortion, and Electrical Aspects. In addition, we've added updated results for Isolation, including Noise Isolation - Full Range and Noise Isolation - Common Scenarios. We've added new performance usages and updated the text throughout the Sound tests and side-by-sides.
- Updated Jul 10, 2025: We've converted this review to Test Bench 2.1, which features minor updates to our sound tests.
- Updated Jul 04, 2025: We've converted this review to Test Bench 1.8, which updates our target curve and Sound tests. You can read more about this in our changelog.
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Differences Between Sizes And Variants
The Sennheiser HD 660S2 come in one color variant: 'Black'; see our model's label. If you come across another variant of these headphones, please let us know in the comments, and we'll update our review.
Popular Headphones Comparisons
The Sennheiser HD 660S2 are open-back headphones and are the successor of the Sennheiser HD 660 S as part of the studio headphones lineage in the stalwart Sennheiser HD 600 range. Unlike the HS 660 S, these have a higher impedance and require more juice from a headphone amp to drive them. Compared to their predecessor, the HD 600 and the Sennheiser HD 6XX sibling, the HD 660S2 deliver a warmer treble response, but the bass and mids sound quite alike across all three pairs. Broadening your search to other studio-grade headphones, depending on your tuning preferences, the Beyerdynamic DT 1990 PRO MKII supply a different sound profile with slightly deeper bass and a much brighter treble.
Check out recommendations for the best audiophile headphones, the best open-back headphones, and the best headphones for music.
The Sennheiser HD 660S2 and the Sennheiser HD 650 are both open-backs with similar designs and mid-forward tunings. For most people, the deciding factor will be the price, given that they sound similar and share limitations such as needing a headphone amp. But, if it's the small differences we're considering, the HD 660S2 are a little more stable. They include standard TRS and balanced cables, plus a carrying pouch. The HD 650 are a little more comfortable, but you only get one TRS cable.
Both the Sennheiser HD 600 and the Sennheiser HD 660S2 are audiophile open-back headphones designed for analytical listening. The HD 600 deliver audio more consistently. While they're both tuned similarly, the HD 600 have a brighter treble, which can be helpful in exposing high-frequency audio artifacts in pro audio work. Meanwhile, the HD 660S2 have a touch more bass, which conveys the punch and boom in kicks and basslines better and a warmer top-end.
While the Sennheiser HD 660S2 and the Sennheiser HD 490 PRO share the same manufacturer name, the latter is a product of Sennheiser's pro audio division, which translates to better performance for analytical listening. The HD 490 PRO are more comfortable, and you can even swap out their pads to change the sound profile somewhat. However, the HD 660S2 are well-built with a slightly more stable design. They also come with a case that can help you transport them on the go.
The Sennheiser HD 6XX are slightly better audiophile headphones than the Sennheiser HD 660S2. While both headphones have a similar design, especially in comfort and build quality, the HD 6XX have a clearer treble sound, which some users may prefer, and their frequency response consistency is better. Their L/R drivers match much better too. However, the HD 660S2 come with a dustbag to help protect them when not in use and an additional 4.4 mm balanced audio cable. Depending on your tastes, you might prefer the slightly warmer treble on the HD 660S2.
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