Just my opinion, but you're using way too many things at once to troubleshoot a problem.
I'd start at the most basic. Make a Kemper backup if you're uncomfortable changing anything. Do an Init Globals from the System settings on the Kemper just to rule out any odd settings that you may not recall or be aware of. This just sets all globals back to their original settings and won't delete any rigs. Then try your rig but monitoring only through headphones, no interface or external cabs. If headphones aren't possible or you just don't trust them then maybe connect your studio monitors directly to the Kemper main outs, being careful to see the output level is not overdriving them.
If the problem is still there at that completely stripped back listen, then try a few generic Kemper provided rigs. Not something you've created or edited. Try a few including something at a similar gain level. If the problem is still evident then at least you have isolated it to the Kemper or guitar.
If all seems OK with this testing setup, then move on by adding another variable, like connecting to one of your interfaces (using the same speakers or headphones to monitor) and listening back to the same rigs you tested with. If the problem now comes back, then it must be related to your interface or how you're connecting to it. If OK with the interface, try adding Cubase back into the mix. and so on.
If you can isolate to something specific and easily replicated, then perhaps Kemper Support would be able to suggest a best approach.
It's pretty suspicious that you say it can't be EQ'd out. Even if it doesn't sound like what you want, you should at least be able to cut those high frequencies out.