I have been having the exact same problem with the Headroom Total Bithead (USB DAC/amp). I have been trying to wrap my head around this for a while, and I have come up completely blank. I have some familiarity with linux and, by default, ALSA contains the drivers for USB DACs like the dared, but for some reason the version implemented in OS2008 does not. I have been trying to simply recompile ALSA but have failed rather miserably.
-- Tarag