Damn! Lost another one!
Seriously, though, I'm glad you were able to work out which one you wanted to keep. Of course the "keep them both" suggestion inevitably comes up, but that's not realistic for everybody (probably not for most people, actually), and I know it must have required a lot of willpower to stick with your plan to keep only one once you had both of them in front of you.
At least you were able to try the DT880, and even better, you were able to make a direct comparison to the HD600. Imagine how it would have been if you'd never done this and then spent half your time thinking
maybe I should have gotten the DT880 instead. Maybe at a later date you can pick up another DT880 (maybe a used one, to save some money) if you really want that sound again.
Anyway, happy listening with your HD600. I know it by reputation as a fine headphone, and the traditional competition to the DT880 when they both were the flagships of their respective makers.