I just tried this out, and unfortunately it would seem there's a bug in the software somewhere. With the mono loop set in the X or Mod slot, switching the stack on and off changes the HF of the piezo signal, effectively giving the sound the characteristic of playing through a cab when the stack is engaged, even with the mix at 100% (as though there was an extra cab sim at the end of the chain that was also being engaged). It basically negates the effect of the acoustic-sounding piezo. It's a real shame, as splitting the signal like this would have been ideal for me. More worryingly, it reveals that the X and Mod slots aren't actually 100% independent of the stack section, so any delays, reverbs, choruses or whatever you have looped and assigned to one of the slots will be coloured twice by the stack/cab sim (once as your signal goes past the stack and out of the Kemper's send, and then again once your signal returns from the loop and back into the Kemper before hitting the Delay and Reverb sections).