Is not always easy. You might need to try a few different amps. You might need to tweak eq a lot. Sometimes the rest of the band cannot afford to wait for you until you finish editing rigs.
but if this is the case then you don’t know enough about what you need to tweak to be able to do it without having the Kemper there to hear the changes. How can you TRY a few different amps or tweak EQ a lot without hearing the results. It just isn’t possible.
If you know that exactly what you need to do (say boost 3.2khz by 1.4db) then you know enough to do it quickly in the rehearsal. If you don’t know the exact tweaks needed the. You need to hear the changes (preferably in the context of the band mix) so you need the Kemper to audition the changes. I still can see how this concept would work in the real world.