I am playing Kemper toaster version through 50 W hand wired amp via Receive input (Jean Pierre Le Roux Amplification made in SA) and through 2x12 Mesa cab, 160 w .
I can broke all my windows and part of the wall in my studio when played only on half volume on Kemper and say 50-60% % volume on the amp.
Never tried 100 % volume on both , since I will probably demolish the rest of my house including my old ears.
During a rehearsal gigs in my garage I play maybe on 70-80 % of amp volume, because of very nasty drummer 
, while Kemper is still on 50 % .
Something is badly wrong with your set up.
Check out the routings and Kemper output pages very carefully on the first place (assuming that amp of yours is OK)
Could you elaborate a bit and what exactly you're doing?
Do you use the Kemper purely for fx (would be my guess since you mentioned 4cm)
Do you use any Kemper profiles? Did you have cab enabled on those?
Very few details on your side to actually make something of this. But I'm not sure if trouble shooting is what you're going for since you already sold the amp?
Make sure your effects loop is serial and not parallel. If you have a dial on the back of the amp that says something like "mix" set to to 100% wet.
Give that a go.
Kemper sounds great when you are in a room by yourself. . . and there aren't other instruments cancelling out your frequencies. . . .like drums (especially vocals) and bass. You need to do a little bit of tweaking to get the Kemper to stand out. Be patient. . . the sound is there...
Sounds like you have an EQ problem. The Kemper requires lots of UPPER MIDS... Look at PRE EQ settings......your holy grail lies within knowing how to adjust those......: :-).
