Thanks for your reply! I've a second problem, and not sure how much it has to do with the first.
I was able to find this thread: http://www.head-fi.org/t/593243/schiit-bifrost-cannot-play-192-sample-rate-under-windows-7-through-optical though it wasn't too much help.
Tinkering around a bit, I was able to get J River to play the source bitdepth, but it was through DirectSound/Wave Out mode. WASAPI/Event Style gives me an error about playback (device doesn't support it).
I guess the thing that's bugging me is that I had no such problems with the Magni/E10, so this is more indicative of the optical out, as you said.
EDIT: Everything's been solved... it was a problem in J. River, had to check the box "Present 24 as 32", and it started working.
Edited by vaed - 2/27/13 at 1:01pm