Ah. With the HD650s, I'd definitely look at swapping the DAC first, and then the amp later on down the road. Of course, that's assuming you're not going to be perfectly happy with the setup you have now when you plug the HD650 into it. Things can always be improved, but a lot of the time they don't need to be improved - really your call once you get the headphones.
I personally prefer optical, as it gives complete galvanic isolation from my computer, and (probably just my mind) the USB input with my particular DAC sounds a little weedy compared to the optical input. There really shouldn't be much difference between the three different styles of digital input, though.