I've just tried and I can confirm that it's possible to connect an ASUS Xonar U3 USB sound card to XPERIA SP.
It's working perfectly, just plug & play (... the music). No apps, no drivers.
The best part is that it requires no external power, nothing. It's powered just from the phone. And beacuse it's quite small, I can take it everywhere with me
The card has its own headphone amplifier so the volume level is really high.
I don't have any other USB soundcard to test, but this one tested and ROCKS !