I don't have SPDIF cables, so didn't try this. As for kemper main out, I tried XLR and 1/4" jack outs, no difference. My audio interface only has 1/4" jack outs.
You say, you run the master volume of the kemper at 0? Isn't the sound muted then?
Cannot test today any longer, it's too late and my girlfriend is already going crazy cause of the noise
Also, I'm in the rehearsal room tomorrow with my band, and I can try it through the PA and with a friend's guitar.
So thanks to everyone trying to help today, more news tomorrow