i have a friend who carries his HD600s everywhere he goes, though im sure he annoys the pants off of everyone he sits next to. i just use a pair of earphones if im out and about, but the hd600 can be powered by a phone/player without an amp. id go for the o2/odac and either the sennheiser hd600s or the beyer dt990s.... i dont think the 650 is much different from the 600. if this will be the best setup you have used then you will be really happy with it either way.
another option to consider is the grado 325is. i would try to test them out before you buy if you have that option available - personally i prefer the grados, but theyre not for everyone, and they certainly arnt nearly as comfortable. they would pair well with either the beyers or sennheisers as a 1-2 combo as they are quite different, if you either hold off on the amp/dac or sell some of your other headphones to raise your budget.