Ok, I felt the need to prove your point, WOW, the scans through the spdif area mess!... I've posted the files below so other people can see... I suppose a cal file COULD be made from these, but I doubt it would be any good...
What I did was, for argument sake, was feed the analog line out from the sound card to the receiver's analog input, and take the receiver's preamp output back to the line in on the sound card, pretty straightforward, here's what it looked like:
bode, soundcard line to rcvr preamp to soundcard line.jpg
then I took the coaxial spdif output from the sound card and fed it to the same (but spdif) input on the receiver, again taking the receiver's preamp output back to the line in on the sound card... i got similar results using the optical out from the sound card, so I wont' post that picture...
bode, soundcard spdif to rcvr preamp to soundcard line.jpg
QUESTION: is this what you'd expect to see, or is this showing a feedback loop?