IMO, if you settle on the UltraMicro Stack, you'd have made a great choice and can't go wrong. I currently use it with my HD650's/cardas, and I'm deeply impressed with the stack. My upgrade path was quite gradual where I started with a Fubar III, then a (SpitFire DAC with HeadRoom MicroAmp) Combo. The sound signature along the way has been remarkably similar, but with each upgrade I've been greeted with more sound detail and separation, especially with regard to base response.
My only misgiving is the 1/8" connector for the headphones, clearly a tradeoff for the compact nature of the stack. You have to use an adaptor with a pair of high level cans, and I strongly suggest the Cardas 1/4 to 1/8" adaptor. However, this may well be offset by the increased flexibility with having the DAC and Amp separate.
I hope that you'll not mince with the powersupply and upgrade that from the stock supplier. That improves the sound considerably when listening to the better and more dynamic recordings.