You might consider solutions other than USB.
Our PDAudio-CF card is a CF card that accepts coax and optical S/PDIF (and
other IEC-958 formats). It's bit for bit accurate up to 24/192. It comes
with an ALSA driver.
For a front end, you might consider our Mic2496. It's a portable 24-bit
dual mic pre-amp and A-to-D converter that provides phantom power and runs
on a single 9 Volt battery. It will sample at rates from 32 to 192 KS/s.
Please see our Web site for more details:
www.core-sound.com
And if you want to stick with USB you might consider M-Audio's MobilePre
USB. (I'm not certain about the availability of a Linux-compatible driver.)
Len Moskowitz
Core Sound