As others have said, you need to strip down to the Kemper and the speakers only. Everything else turned off.
Totally agree. Just to expand a little on what RosboneMako described-
Plug your HS8 monitors directly to the Kemper outputs. Turn off everything else - computer, monitor, interface, etc. Just test it with your guitar into the Kemper with the Kemper out to the HS8's directly connected. If the noise is gone you at least know it's related to the computer/interface. You might also try the same with just headphones, bypassing even the HS8's.
Also, you mentioned no noise with just the guitar amp. Have you tried the Kemper into the guitar amp with nothing else on? That might also help you isolate the noise.