I have both the 6XX and the 660S2. The 660S2 has far better imaging and bass. The 3D layering is fantastic. The bass is quick and deep, textured, detailed, and revealing. On Beck’s “Loser” I always heard a slurring bass line on my 6XX. On the 660S2, I can make out the individual notes. The lead vocals on the 6XX stand out beautifully from the rest of the music, which tends to blend together into one of 3 blobs. On the 660S2, each and every voice and instrument is completely separated from the rest, and the circle of sound around your head is complete and accurate.

The 6XX is more relaxing, more beautiful, with slightly better mids. It is more natural and more neutral. The 660S2 is more V-shaped and energetic. They’re both great with classical, jazz, and classic rock. The 660S2 adds pop music to that. Neither is ideal for hip-hop or edm, or anything that requires a bass cannon that can throw 20Hz hits at you. The 660S2 is reliably good down to 30Hz. Stand up bass viols sound natural, not like the lowest notes are accentuated. The 6XX is reliably good down to 40-50Hz.

I use the 6XX for quiet background music while I’m working or on reddit. I use the 660S2 for critical listening. The 6XX is never quite breath-taking, whereas the 660S2 sometimes is-- even at low volume. The 6XX is always makes comfy sounds. The 660S2 can be distractingly good.