To me the HD-598 is just more fun to listen to and not as relaxed with it's signature (but still smooth sounding). Sure the HD-600 has better and more bass (to some), but I'd rather take the mids of the HD-598 over that. I also felt the HD-598 had much more treble than the HD-600 with stock cable. The HD-600 is good, but it just tends to put me to sleep.
I don't agree that the HD-600 has more detail with stock cable. Heck the HD-600 can barely even pick up recording hiss and is MUCH more forgiving of source quality. HD-600 certainly is no detail monster and I don't think it was ever meant to be by Sennheiser.
Soundstage of the HD-600 is probably more accurate compared to the HD-598, but it's larger on the HD-598. Not even close.
Ok, this is where it gets strange. The HD-598 could be a tad more clear, but at the same time it's not as muffled sounding as the HD-600. HD-600 was very, very slightly muffled sounding. I don't know why. For me, it took specific music to tell the difference in the sound clarity. The HD-600 does improve with better (cheap) DIY cables.
Neither of all 3 are better than the others. Just need to find the one that's best for your preferences and music.
I find the HD-650 far better than the HD-600 for vocals, but I do prefer the more neutral sounding HD-600. Not over the HD-598 though.
A recabled HD-598 really makes it pointless to own the HD-600 for ME. Wasn't worth the extra $125 to me. HD-600 is a good headphone though.
BTW I could never EVER EVER describe the HD-598, HD-600 or HD-650 as being "crystal clear". Don't think they're meant to be. That's more like AKGs/Beyers area.
HD-598 makes me believe that the old Sennheiser "veil" is now a thing of the past. Sennheiser should really work on the sound clarity thing.
Despite loving the HD-598 it could be a lot more clear sounding for the price. It does clear up with better cables. I use Belden 1192A on mine.