I should say that the 600 seems to grow on people more than any other headphone that I am aware of.
Having bought the HD600s w/ Corda HA2 at a time when DT880 comparisons started showing up I immediately had some doubts as to whether I had made the wrong choice. Currently my headphone system is given secondary status (so the best stuff goes to my main system). But I'm starting to really love the HD600s. It is simply amazing strings especially as noted. I give the HD600s a better source and I find there's a lot missing when I hook it up to a lesser source. I put in better interconnects and detail, transparancy, just about everything improves (dramatically).
This has led me to believe that te HD600 is more capable than the DT880s (right out of the box the DT880s might be slighty better-based on the reviews here) but since we're audiophiles we strive for the best and I just don't think the DT880s are capable of that right now or will be in the future. What I mean of course is the fact that the HD600 can be improved with cables. Headroom's XLR setup comes to mind.
But I'm happy to know that the headphones are capable or much more (and my HA2 for that matter). Interconnects, powercords, powerconditioning will clearly make a difference.
Right now I have this setup
HD600->HA2->Sony XE670->PS Audio UO
I use decent aftermarket powercords. I have settled on the XE670 (my 555ES is significantly better) and it's sacd playback is pretty good.
Anyhow the addition of the UO lowered the noice floor by quite a significant amount. I'm gonna be sad when I move it to my main system (can't afford more as of right now) along with my best interconnect. But all this has prompted me to seriously looking at getting the Cardas cables. The HD600s are really good and can be anyone's reference.