Sennheiser HD 660S2 - Wired Audiophile Stereo Headphones with Deep Sub Bass, Optimized Surround, Transducer Airflow, Vented Magnet System and Voice Coil – Black (Renewed)

Straight up, I didn’t think these were going to be mind blowing and I was right. They sound great, they have excellent clarity, and the bass is just right as long as I use the portable amplifier I bought with the last set of Sennheisers I bought.Everything I said above changed once I started listening to Simon and Garfunkel the other day. The tongs and dings and strings all came through like I was listening to that stuff for the first time. The Beatles and Led Zeppelin come through great. That includes the remasters and the Beatles remixes. These are not for every type of listener or music types. Rap and hip-hop are actually listenable with these. That stuff sounds best when the bass isn’t over powering as with some of the Studio Beats I’ve heard things on.These headphones have a totally immersive sound. Bise and Sony might have wireless and all that noise cancelling junk, but these Sennheisers open backs are superior. Those other brands are mass market d and produced. While they’re very high quality, they have a muddy sound compared to these Sennheisers. Noise canceling destroys the clarity on things. I don’t understand why people like that feature. These Sennheisers are not for making phone calls. These are for listening to music, not hearing it. Plug these into areal stereo at home and be blown away by the power. Once I did just that, I was finally overwhelmed.

The first 600 series Sennheiser that have bass! They are not mids focused only. They have low end and the treble is extended further while still sounding smith and never tiring. I always wanted a pair of Senns but their lack of base always turned me off. Not any more. I got them reconditioned and they look brand new. They to benefit from 48 hours burn in. Out of the box the bass was a little muddy. Now they sound perfect. My new daily drivers. Selling my Audeze LCD-2C. I used my fiio k11 for most of my comparisons. I also tried these on my fiioKA3 dongle dac which powered these list fine with no real sound change over the K11. My Audeze LCD-2c have a better bass response but not by that much and I prefer the 660s2 midrange and treble. It's all there but no fatigue what's so ever where the LCD2C can get a little fatigueing after a couple of hours. At $347 for refurbished these are a steal. At $599 regular price you bring in a lot of really good headphones.

These headphones are a lot of things, but let's start with what they're not ...1. There is no noise cancellation, neither passive nor active, and they almost feel like permanently on pass-thru.2. They're not wireless, and that's on purpose, which also makes them a dedicated experience3. There is no signal processing whatsoever in the headphones, they're 100% analog4. The design is mostly plastic and does not feel as solid as AirPod Max or B&W Px8.5. They don't travel well as they don't fold flat. There is a pouch but no travel case available.6. There are tiny eye-straining indicators for left and right, you're better off learning that the Sennheiser 'S' is located towards the back7. At the MSRP they're not cheap, even if far from the highest price point in the audiophile categorySo why bother? In a nutshell: because some limitations come from the focus on audiophiles. You want to take the next level; you have to make sacrifices, apparently. The S2 is a slight update (tweak) on the venerable HD660S. It's not a fast-moving target and the S2 is already about 2-3 years in the market. To call it "old", however, in such a mature market would be simply unfair.Noise cancellation would alter the music in order to counter the noise and here in audiophile land we can't have that. Just to be clear, I do not know if I am truly an audiophile or just imagining it, but I felt the sound slightly improved over my AirPod Max (2024) and even the Bowers&Wilkins Px8. Both are wireless ANC headphones and signal processing potentially is likely to blame for the ever so slight audio distortions. Some songs seem to bring that out more than others. But the ANC allows to focus on the music in noisy environments and that's a win. The HD660S2 simply add music to the environmental noise, and that alone makes them only useful in quiet places (like home). Btw., the open design also makes your music audible to other people in the room, unlike closed headphones. Not super loud, but noticeable.With the HD660S2 being fully analog, the digital codec limitations are non-existent that the internet gets upset about when taking Apple devices out of their ecosystem. I don't want to add to the rhetoric here, but want to simply state that I do find that the discussion often turns too technical based on some numbers only and not really the true sound quality, but that's an emotional topic.Anyway, because the HD660S2 is fully analog (or traditional), and many digital devices moved away from even providing a traditional 3.5 mm or 6.3 mm port, you may need additional hardware. In my case, I got an external digital-to-analog converter, or DAC (FIIO KA15), to attach to the USB-C port (of virtually all my devices). The Sennheiser has two cables, and an adapter included. First, they sport a cable with the substantial 6.3 mm plug. This is an unbalanced line, and the cable is fairly substantial, too. The adapter cable reduces the 6.3 to the more common nowadays 3.5 mm. Of course, audiophiles may protest as the larger plug is still common in higher end devices, but both are unbalanced. A different cable is included with a 4.4mm plug, which is for balanced outputs. In a nutshell, you get both with this headset, and I am currently using the latter as my DAC supports that, too.The DAC is also a gateway to sound improvements by virtue of over-sampling. Say, you have audio source which only provides 44.1 kHz (CD quality) sampling rate, the conversion to analog and associated over-sampling really smoothen out the audio quality, at least enough that I as a mere mortal have a really hard time hearing a difference between a 384 kHz stream going into the DAC and a 48 kHz. I do, however, hear it clearly when looking back at the price tag. ;-) But all kidding aside, a good DAC with excellent over-sampling can take the focus off the hi-res audio source concerns, at least for most of us. The HD660S2 is taking thankfully anything it's being offered (as analog).Note: The HD660S2 are relatively high impedance with 300 Ohm. Some outputs will not produce sufficient volume with that and a headphone amplifier or DAC (in my case that offers a high impedance balanced output) is likely necessary.Like any headphone you just spent a lot of money on, these are the best I ever owned, and it will take some time to actually hear limitations, if there are any. From my current perspective, these headphones offer an amazing sound, provided they're fed with a high-quality signal. Yes, that's at least another $100 for a DAC, not just because most digital devices don't connect to the Sennheiser without it. Even if they did, a "cheap" device (i.e., old iPod) has unlikely the quality converter and over-sampling to take advantage of the HD660S2.On the plus side, the factory refurbished headphones are a great way to save money and $320 is still not cheap, especially considering that there are no fancy electronics, but it's certainly better than the nearly $700 MSRP. It's factory reconditioned by Sennheiser and I did not see or feel anything that showed these to not being new. Yes, these feel like new ones, at least the ones I received.The wired connection via 3.5, 4.4, or 6.3 mm plug is certainly a bit of a limitation in terms of flexibility or mobility, but mentioned DAC did open up a plethora of digital devices in my household. The laptop is the only one having both USB-C as well as traditional 3.5 mm audio, but the DAC livened up the music a bit. None of my iOS devices have a 3.5 mm output anymore, and the DAC connects right to the USB-C. Apple just added support for hires lossless audio via USB-C, but the app decides what sampling rate to use, unlike the Windows laptop where the system sets it (up to 384 kHz in my case). Many songs in Apple Music (even the ones labelled lossless, and some with the hi-res label) are limited to 44.1 kHz (and top out at 192 kHz anyway). Not really the deal breaker for me, as what comes out of the DAC and connected HD660S2 is simply amazing. (Sure, that statement may disqualify me from the Audiophiles illuminati, but it's my honest opinion.) The Nintendo Switch 2 works very well as well, although I am using the DAC also because the Switch doesn't have a 4.4mm balanced output, which I chose for the Sennheiser. Same for the PS5 Portal.Overall, with all the inherent limitations from the focus on an audiophile niche, it doesn't really matter as much that the HD660S2 doesn't have even the slightest form of noise cancellation. You're unlikely to travel with it, anyway. You get rewarded with smooth, rich sound when using a good DAC, and that's all that matters. Well, almost, as the lightweight headphones felt a bit cheap. That impression vanishes when using them, but it does not feel like $700 devices in your hand when comparing to the AirPod Max or B&W Px8.

Sounds great! Better than my Bose QC35 II, Sony WH-1000XM4, and much better than my Focal Elegia (waste of money) headphones. The only problem is this refurbished unit came with someone's strand of hair and did not come with balanced cables. Other than that, I can't complain too much as I did save over $140 bucks not buying new. Definitely recommended.You'll need a good headphone amp to get the most out of these headphones. Without an amp, the volume is pretty low as it is a high-impedance headset. I can see why people are returning them -- they probably are not using it with an amp, and does not sound good without one.