This can also effect the tone.
Clean Sense is the parameter in the Kemper which is responsible for that. I have to say I don't think this workflow is super handy because you need the clean sense for two different types of applications in the Kemper. One is for playing live and to adjust the balance between clean and distorted sounds and the other is for recording or providing a DI track. And in my very personal opinion it misses a dedicated control for adjusting the DI output level. Personally I use several different DI boxes in my studio for different sounds and different applications which gives me the maximum control over the signal.
Yes it would be nice if the spdif volume affected the direct output AND the processed output or the direct output had its own dedicated output knob. Kinda surprised the kemper guys haven't thought of this.