"Firewire devices are (generally) more recommended than USB. Firewire is not "processor dependent" like USB is, and usually experiences much fewer conflicts with the computer compared to USB." Quoting an article I have here on reviewing most mainstream audio interfaces. This is why you don't see many interfaces using USB to record or playback large amounts of audio.
There are options though if you do what to stick with USB that are alreay put together. First off you can get a USB to ADAT converter and an 8 channel line mixer, with preamps to boost your output voltage. Or you can get somthing like the MOTU 828 seen here http://www.sweetwater.com/store/detail/828MkIIusb/
My thoughts on this are the drivers are going to be the most important thing. I know that our EMU has the ability to take the windows output and send it to one of 20 channels doing whatever processing you want to do to it individually isolated to that single channel. You can also route this to most high end software DSP's right within the built in patch mixer.
I think you are looking at major, major, software issues along with limited bandwidth and processing power to run USB. If you are going to bother getting all this stuff together, why not just upgrade your PC a little and run a PCI based card that does everything you want it to do, and boost the output voltage on a pre-existing unit? They do exist. Behringer makes a 230 dollars ADAT mixer that comes with 8 preamps with individual outputs. I doubt you can get 8 volts, but something like this seems like a more viable solution.
Why re-invent the wheel?