Digital Interface as stand-alone mixer
 

Is anyone using a digital interface as a stand-alone mixer? I first noticed the SD USBpre had S/PDIF I/O and then started looking at other firewire and usb digital interfaces that had S/PDIF I/O. I am thinking of using my M-Audio Microtrack II as a digital recorder (S/PDIF in) with a digital interface as the mixer.

I haven't been impressed with the mic-pres on the Microtrack II, so I'm looking for solutions to make it more useful as well as a digital interface at the computers. Firewire is preferred with its lower latency. The only interface I've gotton my hands on is the Tascam us144. Its mic-pres probably aren't any better than the Microtrack's.

Budget is a consideration, otherwise, I wouldn't ask the question and gone straight for an SD 788T or better;-)