I'm reposting this from the problems forum in case it helps someone else.
Using SPDIF out on the Kemper into my DAW sound card I got clipping with most profiles without changing anything, much much too loud. Here is what I found:
Inserting a Gain control on the SPDIF input (in my soundcard control panel software)allowed me to remove all clipping with all profiles. I had to drop the Gain by -11db(depending on profile). After this the Kemper sounds perfect. Exactly like the XLR outs. I didn't even know you COULD have a gain control on a SPDIF.. I probably don't understand SPDIF. I figured that a device cant send a SPDIF signal passed +0db but in this case I must be wrong about what is happening.
Also, it helps to max the Master Volume even if you use the SPDIF (and the Master has no effect) because you can see if the output levels are clipping via the Clipping light.
PS thank you C.K., your advice about changing gain controls along with statements about dropping overall volume levels by 6db got me on the right track.