Well, for the moment anyway, we're at the end of the debugging chain in the studio. I just swapped the optical cable coming out of the back of the d8b and the failure followed the cable, not the card, so I've got a bad optical cable. Possibly two bad cables, because it's not clear that the return cable is good, but I can't test that conveniently at the moment.
I'll drop Bob a note and order a couple of new cables.
In the meantime, I have swapped in the new video card and am hoping that all things remain stable with it. I'm downloading the latest drivers for it now.
So the only thing that I'm still waiting for is the monitor speaker (and the optical cables, of course).
I'll keep my fingers crossed that nothing else is wrong...