I had the 660S2 for a couple weeks, by far the most audiophile headphone I've had (Senn hd280, Sony wf/x 10003/4, b&w px7, beoplay ex, you get the idea). Blew me completely away. Never heard any of the 600 series, so bought the 660s just to a/b. Listened to two songs and put them back in the...