I just picked up the HD600 Yesterday and own the X2 as well. I think they are completely different sounding. Both are good. HD600 has better mids and a smooth top but lacks the low end of the X2. I think they have a spot for different types of music. I listen to a lot of progressive rock so the HD600 may work better for me in the long run. I however will keep both.
5/12/15 at 5:45am