Now that I think of it, as long as the DAC will do high-res through coax, won't the M2Tech HiFace (or whatever it's called) convert USB to high-res asynch coax? If that's the case, there are far more options. James Tanner used one on his Bryston BDA-1 when he was demoing it at a show I believe. If that's the case, it says a lot, as Bryston wouldn't use any piece of junk to show their gear IMO.
The Arcam rDAC has asynch USB licensed from dCS. The rDAC has gotten great reviews, especially for it's USB input, and is $479. Haven't seen one second hand yet, so I'm pretty sure it'll command most of it's full retail if it doesn't work out.