I have a NuForce ICON HDP that I use with my HD 650s and I enjoy it. I lack the substantial experience and/or knowledge base with DACs/amps to really understand whether or not it's a good pairing, but it doesn't sound terribly different from the ASUS Xonar Essence ST it replaced. From what I've read though, it seems that the HDP is a better DAC than it is an amp.
I've also been looking at tube amps since I see "Senns love tubes!" repeated so often in different places around here. So far I've been looking at the Schiit Lyr, but I'm kind of afraid it will fry my lower impedance cans.
Ha, Im pretty much in the exact same boat as you are. I have the Xonar ST and think it sounds pretty darn similar to my HDP. Which is a good thing to me as that means what I hear on my computer matches what I hear on my HDP. And I too have read the same about the DAC being the "better part" of the HDP. AND I have been considering a tube amp as well and have been thinking about the Lyr or the Crack. I dont intend to use any of my amps/dacs with anything but high impedance headphones though, so Im good there. Someday I will probably pick up a pair of Denon D5000's too just to have a nice set of closed ear cans, and since they are low impedance Im not sure what Im going to drive them with yet.