The HD600 and DT880 aren't considered that different in terms of technical ability, though they have very different sound signatures (note: some people would disagree). Well, back in the old days when the DT880 had come out but not the HD650, the DT880 and HD600 were considered on an approximately equal level, as flagship cans (excluding the Orpheus) of their respective manufacturers. Nowadays I hear people comparing the DT880 more with the HD650 than the HD600. That may be a function of loss on the HD600's part of flagship status, but it still makes me wonder about how the times change. But in either case, if you prefer the DT880 sound more than the HD600 then the change, which is likely more a side-step than an upgrade, may be worth it, but if you are satisfied with the HD600 then I personally wouldn't find any reason to get the DT880, unless you're just curious and can afford it. If you want a true upgrade, you should go for the HD650.