The HD660S, or the last member of the legendary HD6X0 family that debuted in 1993 with the HD580. It’s said to be the replacement of the HD600 and HD650 (although both are still produced in their new 2019 revision).
The HD660S uses drivers from the HD700 (discontinued) with some light modifications, in a HD6x0 chassis. Same build, pads. Only differences are a Sennheiser logo on the grills, not present on the older models, and a lower 150 Ohm impedance. Build is solid with durable plastic, and comfort is great, despite the stronger than average clamping out of the box.
○ Release : 2017
○ Current status : In production
○ Type : Over-ear / Open-back
○ Measured weight : 268g
○ Impedance : 150 Ohms
○ Sensitivity : ~98 dB/mW
○ Average used price : $280-330
The HD660S is quite warm. Generally, the HD660S is smooth and inoffensive, with solid natural tone, just like its precedessors. I still have the HD600, HD580 and I’ve owned the HD650 (same as HD6XX). Of the 4, the HD660S is the most relaxed in the upper-midrange (around 2-4 kHz).
Bass is very similar to the HD580/600 : slight mid-bass and lack of bass extension below 60-70 Hz. The midrange has great tone and will be preferred by many people over the other HD6X0 models it’s a few dBs more recessed. However, the treble is a bit a mixed bag. It doesnt sound sparkly or engaging to me. Quite noticeable lack of air above 10 kHz. The others HD6X0 have better air clarity than the HD660S. The HD660S will often sound a bit muffled next to the HD580 or HD600 because of the more dipped upper-mids and very quiet upper-treble.
The HD660S appears to be just barely cleaner sounding than the others, with improved dynamics – mostly noticeable in the bass and midrange. However, I can’t manage to hear big differences in imaging / staging between the HD660S and the HD580/600.
The HD660S would be nice at $300, but not at MSRP $500. I prefer the HD580/600 because of the better treble and slightly brighter tone. Both HD6XX/650 and HD660S are just a bit too polite in the treble for my tastes.
Measurements & Resources
Typical HD6X0 serie bass roll-off. Yeah, a bit disappointing for an open-back released for $500 in 2017. Midrange is more dipped around 1-3 kHz than the older brothers. Quite smooth treble, even with the minor 8-9 kHz energy. Hard to notice the lack of upper-treble.