A high gain amp works by reducing the lows, drastically raising the gain(amplify), then apply some post EQ to bring it back to what it should sound like. If you dont remove the LOWs, the bass will dominate the distortion and go from flubby to a fuzz pedal.
Flub comes from too much bass entering the gain circuit. Meaning the bass is too high BEFORE the amp section.
The Kemper has a DEFINITION control in the Amp section that reduces the lows coming in from your guitar (before the amp). Most Marshall profiles will want the DEF around 6-7. Higher gain profiles may need to go all the way to 10. This also depends on how the profile was made. Sometimes the profile is a mess and has too much bass when it was made so Def needs to be at 10.
If that does not help enough, you can use a Graphic EQ in slot #1 and reduce the bass values. The GEQ will also let you boost mids a little or do some high freq cutting to tame the highs.
I would suggest starting with the DEF at 5 and going up slowly. At some point the guitar may start to sound clearer (it may still be too bassy). Then adjust the GEQ to remove the incoming bass.
A high gain amp may setup the incoming guitar signal to have a freq response similar to this. As you can see the LOWS are all being turned down to reduce flub/mud/bass/rumble. So using the GEQ to lower 64 Hz(-3dB),125Hz (-1.5dB),and 250 is where you want to adjust.
NOTE: I also like to put the noise gate AFTER the GEQ since lowering EQ values reduces some noise itself and makes the gates job easier. YMMV.