Bump,
same issues here, and have also done all of the same fix attempts as OP.
I did also try the 1394 Legacy drivers from Windows 8.1.
I’m dead in the water. Drivers all appear to be in place, but I get jack.
NOTE: The interface still seems to work physically. I have a keyboard going into it, and it still sounds as grand as ever.
Possible Driver issues beyond customer abilities Steinberg?