If you go with Sennheiser, the new model, HD650 is a big step up from the HD600 or HD580. I agree that the the step from 580 to 600 is small, but the 650's are truly world class, but now you're looking at $350 and then it would be a shame to not amp them properly with something like an Emmaline XP-7 ($500 headphone amp) or a PPA which is a DIY amp (see jmtaudio.com) for a little less. It's all a matter of whether you want to jump in toe first or head first (no pun intended, actually) and what sorts of applications you'll be using the cans for. Head-fi is the place to go because you will get all sorts of ideas about what you can do with any budget.
