It wasn't an acquired taste for me, the second I put on a pair, I knew I had to have some. Don't have any yet but it is tax refund time and I am ready.
As for the HD650, I flip back and forth between that and my HD600's I would have to say that in fact, whichever pair I have on is my favorite at that moment. I like the 600 for critical listening but the 650 does something to the music I can not describe, makes it more enjoyable is all I can say. I have one other headphone that does the exact same thing, and that is my HD580's.
I guess I'll know when I hear them :p. I had both the HD600 and HD650 simultaneously for a while and ended up liking the HD650 for that extra thump and extension. That and I didn't have quite enough music that was of 'fantastic' production quality. The HD650 were more forgiving.
Yeah, the 325i's were nice and thumpy. I had no complaints on bass, but it could get shrill depending on the music I would play with it through the Asgard 2.
Vs the 600: the 650 is slightly warmer.. but is more resolving, possesses a larger soundstage (in depth, width, and height), boasts better extension (in both directions), features noticeably better dynamics and texturing ability (especially through the bass.. and that's not the slightly enhanced midbass talking, btw).. a midrange that I found even more captivating than the 600's midrange.. and despite the 650's treble being (comparatively) shelved down.. it's smoother, more detailed through the upper frequencies.
That's the magic, bud.
I found the HD600 to be slightly more open and airy in the treble than the HD650. Could be the difference in sources and amping, but bass is by far spot on.