You're missing an important detail, devices like that won't carry a Dolby Digital signal through. Digital signals will almost always be converted to PCM, losing Dolby Digital, losing the ability to let the Mixamp convert into Dolby Headphone. That, and it's doubtful the USB even takes in anything more than 2 channels.
In short, again, Mixamp must go first.
Thanks again, I keep jumping the gun a little...
Anyway I think the solution has been staring me in the face all along, I was planning on buying the Fiio E17 as a replacement for my my Asus Xonar U3. But I'm probably better off just buying the E17 for general use and then keeping the Xonar for when I want to play games as that supports Dolby Headphone/Dolby Live natively etc.
Might be interesting to see if there's any difference in quality when plugging my PC360's directly into the Xonar U3 for Dolby Headphone, or using the spdif output on the U3 carrying a Dolby Digital Live signal to my Astro Mixamp.