Hi Simon,
similiar problem here. I can adjust the sound NOT coming from the SPDIF channels perfectly, but i have a similiar issue:
The two tracks coming from the KPA via the SPDIF channels (in my case called 3 and 4) have a totally different input to my DAW.
The clean is not distorting like in your case, but much much louder than the rig which is recorded in the other channel, to such a degree that i have to normalize the rig sound.
it is not audible in my project otherwise, which is fortunately not a big issue, because i have the reampable track right.
Nevertheless a strange issue. Why produce those two channels such a different volume??
Greetings from Germany