I generally tend to set the volume of the SCU down a bit, as otherwise you can't even get off green and it's really loud. I give the volume down button on the SCU a few clicks and then go just into the blue colour on the volume control box. I don't mess with the default levels for each channel, I find them just fine and I hear a full 360 differentiation between noises very clearly.
Personally, if I was you mate, I'd stick with what I'd brought and not use the soundcard any more, just rely on optical from now on and the SCU to decode the digital signal.
Whether one is better than the other though, I really wouldn't know, what I do know is that the SCU/headphones combination is truly stunning.
Be warned though, you can even have them really quiet and you just can't hear a thing going on in the room, winds my missus up something chronic when she's trying to get my attention, then she taps me on the shoulder and I mess myself 'cos I've been in the middle of a tense bit of AVP lol