Hi, I am also using a Kemper Stage with the Line 6 plus powered Cab. The sound is incredible, but I have not been able to get a decent volume out of the Cab. I am not sure if I am doing something wrong with the settings on either the Kemper or the Cab. I tried by changing the volume levels in the Cab and it improved a bit, I also tried the output volume in the Kemper and the issue is that if the volume is too high the Powered Cab starts showing a red LED and after that the sound gets distorted. I play it usually at home so I am good but once I try this on stage as a monitor I might need more juice out of it.
Again, i am extremely happy with the sound quality of this combination, it sounds pretty close to the real amp. I use a lot the different cabs from the Line 6. I disable the cap from the kemper and get absolutely beautiful warm tones out of the built in Line 6 amps.
My favorite combination is the Fender Deluxe Twin Amp + Line 6 Vintage Cab + PRS Mc Carty bridge pickup (warm tone). It sounds supper Jazzy.