Tested: Sennheiser HD 660S2 Headphones – wonderful hi-fi sound with brilliant mid-range response

The Sennheiser HD 660S2 audiophile-grade headphones launched earlier this year and it’s time to put them through their paces for review. I am, admittedly, not overly picky when it comes to these sorts of hi-fi cans. I can certainly appreciate the sound quality, but not I’m not type to spend thousands on a pair of cans and that’s where the Sennheiser HD 660S2 come in. They, at least for me, seem to hit the sweet spot between really high-quality sound reproduction without being so expensive than I just can’t bother caring all that much. And I’m happy to say I’ve come away (mostly) quite impressed with the experience thus far. Head below for a deep dive into our time with the Sennheiser HD 660S2 headphones. 

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Hands-on with Sennheiser HD 660S2 Headphones

Sennheiser’s HD 660S2 headphones are audiophile-grade cans made for critical listening and those that take their music seriously. The high-fidelity sound reproduction comes by way 38mm drivers, an open-back design, aluminum voice coils, and a vented magnet system, some of which you can get a look at via the see-through grilles on the outside of the ear cups. The made-in-Ireland cans feature a 300 ohms impedance rating – they work best with something that can really drive them as opposed to your average smartphone or MacBook. 

The matte black frame is joined by hits of copper lettering and the Sennheiser logo while padding sits under the headband. The earcups feature particularly plush velour treatments that have been a joy to rest on my head for long listening sessions. 

• Uniquely powerful vented magnet system for minimized distortion
• Ultra-light aluminum voice coil with 300-ohm impedance for excellent impulse response
• Intimate sound with great timbre and deep, accurate sub bass
• Plush ear cushions and open design for maximum comfort
• Outstanding quality designed in Germany, made in Ireland

They ship with a 6.3mm cable (1.8m), 4.4mm cable (1.8m), and a 6.3 mm to 3.5 mm adapter, alongside a carry pouch. Regularly $600, you can land a pair on sale for $449.99 shipped (at the time of writing). 

Here’s a closer look at the specs:

• Transducer principle dynamic, open
• Transducer size 38 mm
• Frequency response 8 – 41,500 Hz
• Impedance 300 Ω
• Sound pressure level (SPL) 104 dB (1 kHz, 1 Vrms)
• Total harmonic distortion (THD) <0.04% (1 kHz, 100 dB)
• Ear coupling Over-Ear
• Weight 260g
• Adapter 6.3 mm (1⁄4 inch) to 3.5 mm
• Connector 6.3 mm (1⁄4 inch) jack plug, 4.4 mm balanced plug
• Cable length 1.8m
• Magnetic field 4.5mT

9to5Toys’ Take

I was on the hunt for a new hi-fi set of cans this year and the Sennheiser HD 660S2 Audiophile Headphones seemed like an obvious candidate in the price range. I had always heard good things about its pro-grade open-back headphones and came away impressed after spending a number of quality listening sessions with them. They, at least for me, deliver good sound quality across a wide array of genres – an important trait for my listening tastes – from the hardest trappy hip-hop and bass heavy EDM tracks to particularly organic and delicate recordings, they did the trick for me. 

Now on to the build and comfort factor. The plushy velour ear pads rest nicely up against my head, creating a solid seal without being overly tight. They do squeeze slightly, but also seem to have already started to ease up just enough while maintaining a solid seal after a few weeks of longer listening sessions. 

The headband band isn’t quite as generous in terms of cushioning, but is still quite comfortable and hasn’t caused any issues for me. 

All-in-all, its a comfortable experience even after wearing them for a few hours straight working on Logic Pro sessions and listening to albums. 

The actual frame of the headphones is your usual matte plastic treatment – nothing overly robust or special here. The plastic looks nicer than it is but isn’t so cheap that it’s going to snap or fall apart any time soon as far as I can tell. It also, presumably, allows the headphones to deliver a more lightweight wearing experience (about 260-grams).  

The see-through outer ear cup grilles are a nice design touch, showing off some of the innards in the process and adding a bit of interest to the aesthetic vibe here.

Sound quality is always such a subjective thing, so it’s hard to really write down on paper why you will like these or not. But I think I can confidentially say they deliver an overall balanced experience that treats a wide scope of musical genres with respect. 

The bass reproduction on more dancey and poppy tracks is mildly lack luster – it still sounds great but it’s not as aggressive and punchy as you might hope. They aren’t overly subby (in a good way), but they also aren’t particularly upfront and tight on the low end – folks specifically sensitive to that sort of thing might want to look elsewhere. Although I should say they do, without a doubt, outperform the more trendy headphones I have around the office in lower price ranges in this regard (as they should at this price). The bass response certainly isn’t bad, and quite possibly the only thing stopping them from being a total 10 out of 10, instead of just a 8 or 9 out of 10 for me. The bass is just fine, but not perfect to my ears. 

But I also think that highlights where Sennheiser really hit it here – for lack of a better way of saying it, they are just easy to listen to. The high-end isn’t too bight and aggressive for my taste and the bass is balanced and weighty without getting in the way of what, for most folks, really counts; the vocals. 

For me this also meant that, for songs that are particularly aggressive and noisy in a good way, you can really crank up the volume without things getting overly harsh and jarring – think wild industrial music in the vein of classic Nine Inch Nails, noisy cymbal work on heavy rock, and wonderfully grating EMD synth design.  

Where the 660S2 set really shines is in the mid range – vocals sound fantastic, natural, and right in the sweet spot for most genres to me. The same goes for classical instrumental arrangements that fill up these frequencies in lieu of a vocal performance. 

There’s a particularly enjoyable and spacious sound stage reproduction here for me that, in combination with the wonderful mid-range response, delivers a wide and natural feel.

If I had to choose, I’d say the Sennheiser HD 660S2 Audiophile Headphones are best for organic, acoustic music with a strong focus on the most important part of the human voice’s frequency range. Bon Iver and classic singer/songwriter music sounds absolutely spectacular to me on these for example, and, outside of the slightly (and I stress slightly here) underwhelming thump in the lower register, vocal-heavy pop and electronic music does too. 

Buy the Sennheiser HD 660S2 Audiophile Stereo Headphones

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