I'm sure this will have been discussed before, but I couldn't find it. I use a pair of HD 650's daily, and it's been a bit of a love affair. For me they really hit the spot when it comes to bass response and clarity. They have bass, but it's not boomy. It seems like everything I tried before these (which I've been using for more than two years) either had very little bass, or bass over everything else. Either way, the HD 650's are one heck of a great headphone. But the question is - is there an upgrade to an even better set of cans? Let me set a budget of say..... $750. Any ideas?