I have just bought a Razer Tiamat 7.1 and an Asus Xonar D2X together, however I have just gone over the Tiamat's website page again and found the sound cards compatible with the Tiamat, and the Asus Xonar D2X wasn't one of them.
Will they be able to work together in perfect harmony?
As a rule: USB connections mean the soundcard does no work whatsoever. Razer claims the Timat gets around this limitation as part of their driver package, but I can't comment one way or the other. If they don't list any ASUS cards though, then its likely that feature isn't supported.