Well the difference between the EMU 0404 USB and PCI is marginal. The only useful thing in the USB is that it has balanced outs. The 650 deserves a better source than the EMU... way way better. Also of what I heard, i'd rate the HD650 as being significantly better than the HD600. Yes its darker, yes its even more revealing but it just sounds so much more nicer than the HD600 which basically sounds quite similar to my old HD580. Infact, I'd have a hard time distinguishing between the 580 and 600 in an ABX.
The external DAC I use is significantly better than a 0404 when given the right SPDIF source and the headphone is bottlenecked even by that. I tried the headphone with a friend's Bluenote Stibbert CD player and it just completely blew me away. This was with a cheapo home built amp.
I'm currently building a better amp based on the Cavalli Kumisa 3 design with Raghunandan's help and hopefully it should be a lot lot better. IMHO, buying headphone amps for hundreds of dollars is a waste of money. If it turns out all well, I don't mind building a few extra ones for TE members.