VS. the 400: Basically, the 600 can do EDM like a perfectly EQ'd 400 can - without any EQ at all. But then it also excels at genres where the 400 is limited. The 600 excels in classical and jazz as well. This is an all-rounder that actually excels in most genres. If I were to A/B them, I might be able to give the 400 an advantage in bass texture, but I didn't have the 400 with me.
VS. the X1: As I said, Chico basically got it right. Better bass extension and quality, better treble, better soundstage, detail, refinement. I would say the X1 is slightly warmer and I may give the edge to it in some vocal tracks. That might be the only area where the 600 is lacking a bit. From what I've read, the HE-500, with its lush mids, might be the perfect complement to the 600. Then you'd have just about every genre covered. But if I could have just one headphone, it'd be the 600 (or 900). And by the way, these do not sound closed to me AT ALL, besides having potent sub bass that stays just where it should.
Edited by ethan7000 - 9/30/13 at 10:39pm