Headphone Reviews

Sound Quality
On a gross level the HD 660 S sound is similar to the HD 600 and 650. Zooming in a little closer I'd characterize the HD 650 as the warmest of the bunch and the HD 660 S the brighter of the trio. Overall, I'd characterize the HD 660 S as a little lean.

To my ears this comes from two things: The HD 660 S has less of a broad upper-bass centered hump it's virtually flat from 100Hz to 1kHz, and the bass falls off gradually below 100Hz. But I also hear a more forward presence region from 1kHz to 3kHz. To my ears the HD 650 (in stock form) does warrant the "veil" designation; the HD 600 a little; and the HD 660 S not hardly at all.

Imaging on the HD 660S does appear to be a bit deeper and wider, but it's mostly quite like the other two headphones, which to my ears is good reasonably wide, and not particularly deep. The HD 660 S also appears to have more impact. You'll notice I used the word "appear" in both those descriptions. There's a reason for that...

The biggest difference I hear when switching from the HD 660 S to one of the other two, and I heard it immediately, is the fairly significant sense of a smoother and more refined sound of the previous cans. Switching back to the HD 660 S I hear it as grainy more more edgy in comparison. I'm quite saddened by this.

The HD 600 and HD 650 have for well over a decade been considered one of, if not the best headphone available. They are legend! Right on the HD 660 S box it says, "The Legend Continues." Sadly, I think they are mistaken. Right off the bat, this graininess takes it out of the family's rightly earned status.

But it gets worse. One of the defining characteristics of the HD 600/650 is a wonderfully smooth response. I had the good fortune once of sitting right in the circle with the Muir String Quartet and I was simply breathless with the absolutely smooth nature of the sound. Reproduced violins can take on a screechy character; in real life they are not at all. The HD 600/650 does a great job of portraying this smoothness. Very good treble response should give the impression of smooth detail. So many people mistake a bright, sizzly response for good detail. It just ain't so.

The smooth detail of the 600/650 allow them to "scale" well, bringing out the advantages of upstream gear. I tried the the three headphones with my HeadRoom Max, Bottlehead Crack, and Simaudio 430HA amplifiers. The differences were readily apparent with the 600/650, but was much less so with the HD 660 S. The grainy character got in the way.

While the HD 660 S was a bit brighter than the other two, it also lacked the nuanced resolution and dynamic balance of the 600/650. Low level details were lost, and grosser detail seemed compressed into a similar level of response. My measurements show these cans as far more similar than they actually sound to the ear.

To make matters worse, the design of the HD 660 S driver makes them less able to be modified by DIYers. Ugh.

What the...
I just don't understand what Sennheiser was trying to accomplish with these headphones. The intro to their product page says right off the bat:

The new HD 660 S lets you enjoy sophisticated audiophile sound in even more listening situations. The successor of the legendary HD 650 excels with the improved performance of its new transducer design. Thanks to its lower impedance, it delivers reference-class sound also when connected to hi-res mobile players, thus offering much greater versatility.

A) The HD 660 S is only a dB or two more voltage efficient, so there's only a modest difference in volume from a phone. On its product page, Sennheiser claims 104dB/Volt for the HD 660 S; in its manual, Sennheiser say the HD 650 has 103dB/Volt sensitivity. Higher-end portable players will all have plenty of voltage to drive a 600/650 to loud levels.

2) If you're going to tout a product as being more useful in portable applications you should include a short cable in the mix. Both included cables are far too large for that sort of thing; neither are terminated in a 3.5mm plug—for the moment at least, still the most common connector on portable devices. Add the 1/4" to 3.5mm short cable adapter and you have a fairly clumsy connection to portable devices.

I've also seen a post where a Head-Fi made an inquiry to Sennheiser about whether the HD 600 or HD 650 would be discontinued. The Sennheiser representative said the HD 660 S will replace the HD 650 in the line-up, and the older model will be discontinued. I've personally always preferred the HD 600, but acknowledge the HD 650 has a lovely sound many might prefer.

I'm bummed.

Summary
Were it any other maker producing the HD 660 S I would have been quite impressed with how close they managed to get to the sound of the legendary HD 600/650. But from Sennheiser themselves producing it and with plans to discontinue the HD 650 I can only shake my head in disbelief.

I loath the race to lower headphone impedance in high-end headphones to make them more compatible with portable devices. My experience with the three beyerdynamic DT880 versions with varying impedance gives me the strong belief that higher impedance headphones will outperform lower impedance cans of the same design. Unfortunately I think this holds true with the new lower impedance HD 660 S.

The new Sennheiser HD 660 S lacks the legendary smooth resolve of the HD 600/650, preventing it from scaling well as upstream gear is improved—one of, if not the most, important characteristics of a great enthusiast headphone. If you're not getting improvements on money spent on electronics you're missing out on half the fun.

Yes, I'll give these headphones a soft recommendation...they remain a decent headphone—comfortable, stylish, and close to as good as an HD 600. Maybe the stealth black look and smartphone capable efficiency is enough to tip the balance for some. But I'll also steer buyers towards the HD 600—or Massdrop HD 6XX if it becomes available again—as the better sounding alternative.

Video
Watch on YouTube.

Resources
Sennheiser U.S.A. home page and HD 660 S product page.
Head-Fi impressions thread, sponsored thread, and reviews.
Superbestaudiofriends.org impressions thread and 600/650/660S analysis thread.