Look for a nice clean high quality 2nd hand pair imho.
Before you do choose a pair, try and find a store that stocks them so you can at least try them on. Nothing is worse than having a set of cans that 1. dont fit your shape head nicely, 2. dont fit your ears properly, 3. are flimsy as hell - creaking with slight twisting etc.
+1 on the 668b's. I have a pair, and they are excellent after the approximately 30 hour burn in period. I'm also working on a few mods for them, I may make a forum post about the process. I have replaced the acoustic dampener in them with something similar to clay (it's actually the blue tacky stuff for hanging posters) and it has improved the sound a ton. I suggest checking out some threads on the rock audio forums, they have a full list of mods you can do. Regardless, right out of the box these compete with 100+ dollar cans. Get them.