I'd say don't even bother. USB sound might cause you more problems than its worth. The built in soudcard on your iMac is fine.
I second this. I have my A5s connected directly to my iMac's headphone jack and they sound fantastic. In fact, the A5s were designed to be driven by an iPod's headphone output, so adding additional hardware to the mix is only likely to complicate matters while yielding little to no sonic benefit.