I like the Sennheiser HD 660 S headphones a lot. They sound clean, they're extremely comfortable and as high-end headphones go they're pretty affordable at $500, £430, AU$799. The HD 660 S model looks like a near twin of Sennheiser's older, and still available, HD 600 and HD 650 headphones, but the HD 660 S' driver is different; it's more similar to the HD 700's driver. Sennheiser isn't breaking new ground here, it just keeps refining the formula.
The HD 660 S open-back headphones won't do a thing to hush external noise, but they are substantially lighter than the average over-the-ear design, weighing 9.7 ounces (260 grams), and even though they're high-impedance (150-ohm), Sennheiser claims the HD 660 S can be enjoyed with portable devices and smartphones.
The headphones come with two 9-foot-long cables, one terminating in a standard 6.3mm stereo plug (a 3.5mm adapter is also included), and the other with Sony's newly developed 4.4 mm Pentaconn balanced stereo plug.
Build quality feels robust, and the HD 660 S comes with a two-year warranty, double the length of coverage you get with most headphones.
The sound is remarkably open and clear, that's what you hear from the get-go. I spent a good deal of time listening to movies with the HD 660 S, and they really are the sort of headphones that "disappear" once I'm engrossed in a story. Their unusually spacious soundstage gets some of the credit for that, and the sound is low in distortion, which helps reduce listener fatigue as the hours roll by. Also noteworthy is that I can comfortably wear my glasses at the same time, and that's not usually the case with most on- or over-the-ear headphones.
Lucinda Williams' brilliant new remake of her "Sweet Old World" album puts her voice up front, and her voice, with a lot more miles on it than when she made this record back in 1992 only makes the music more heartfelt. She's literally singing from a different place, tempos are slower and the HD 660 S made the connection between Williams and me stronger.
Grizzly Bear's "Veckatimest" album's sound made me smile. The massed vocals, acoustic guitars and drums were all set in a deeply reverberant soundfield on my Astell & Kern Kann portable music player, and the result was flat-out gorgeous. The throbbing bass drums had terrific impact and power. Switching over to a set of($699, £599, AU$999) open-back headphones the sound was a lot more transparent and dynamic, but the imaging was more inside my skull and I missed the HD 660 S' richer tonal balance.
Listening at home with aheadphone amp, the HD 660 S and EL-8 both sounded a lot better in every way than they did with the Kann, but the sound differences remained: The EL-8 was undoubtedly clearer, the HD 660 S was warmer and more relaxed. Hitting the Alabama Shakes "Boys & Girls" tunes hard with the HD 660 S was so satisfying, it was a total pleasure.
Unfortunately, I didn't have HD 600 or HD 650 headphones on hand for comparisons, but there's a HD 700 in my collection. That one's sound hews closer to the EL-8, which is to say the HD 700 is also clearer than the HD 660 S. On the other hand I preferred the HD 660 S' better balanced midrange, so voices sounded more natural with the HD 660 S than they did on the HD 700.
Plugging the HD 660 S into my iPhone 6S the sound was decent enough, and Trombone Shorty's new funked-up "Parking Lot Symphony" album played loud enough, but dynamic punch was lacking. So yes, the HD 660 S is usable with portable devices, but to hear this headphone at its best use it at home or with high-quality portable players.
The Sennheiser HD 660 S is a charmer, it's the sort of headphone you look forward to spending time with when you get home.