The biggest difference is that the HD600 is much more forward in the upper mids around 3 KHz. I'm pretty sensitive in that area, so it makes the HD600 unpleasant at higher listening volumes. A lesser difference is that the HD58X has better extended and tighter sounding bass, though that might mostly just be because I can comfortably listen a bit louder than with the HD600.

It's been years since I've had the HD600, but I would say that on treble quality (extension and evenness) the HD600 is probably better than the HD58X, so if you listen to a lot of acoustic music, especially string instruments, and you don't mind the shout around 3KHz, the HD600 is probably a better choice.