Well ... that didn't take long.

During the time it took me to write the comments above it occurred to me to try some other audio program, so I loaded Audacity, configured it for the C400 and it worked. Then reloaded BIAB, and that now works perfectly also.

I'm guessing somehow the interface between BIAB and the audio drivers got into some sort of hang-up state?

I'll play with this all more to see if I can re-create the problem.

Hoping my issues add insight to others?

John