Could well be the USBPre, but you aren't the only one with troubles with getting a digital signal to lock using the 680. You're having problems, the guy in the 680 thread failed to get a digi signal using 2 different external A/D's, and my friend can't get his V3 to work with the 680, even though his mytek works fine.
As much as I like my 680, all these issues people are having feeding a 680 a digital signal are disconcerting. Hopefully at least the USBPre>680 issues can be tracked down and solved with a new firmware or something.
I've been thinking of getting a V3 or USBPre to go with my 680, and though I've used one borrowed V3 successfully to feed my 680 a digi signal, all these issues make me nervous about plunking down to get a new A/D just to find that it might not work with the 680.