Hi Ajs6593 , The high cut recommendation from GearJocke is a good one, and that can definitely get rid of high end fizz. There's a High Cut in the Output menu if you select the Output soft button. That way you can save yourself an effects block for other stuff I typically set mine anywhere between 8k-10k depending on how much high end fizz or high end unpleasantness I'm hearing.
Another thing I'd highly recommend looking into, is the PureCabinet settings in your KPA. PureCabinet is a feature that's purpose is to smooth out any high end fizz caused by mic phasing and stuff like that. But it tends to change the high end a lot with high gain profiles and can in my experience, either make fizz more prominent and move the fizz to more obvious frequency ranges, or get rid off the fizz and smooth out the high end. It can have very different effects on each profile especially when working with high gain. It's definitely worth turning it off overall and then experimenting with it to see if having it either off or on at various settings can help solve your problem.
PureCab is set to 3 or 3.5 automatically on the KPA and is located in two places, the Output menu and in the Cab menu. The PureCab setting in the Output menu is a global setting, and the one in the Cab section menu is a "per profile" setting. There's also a PureCabinet soft button to turn it on and off globally in the Output menu, and unless it's turned off globally by using the soft button to disengage it, then even at 0 PureCab is still on. I just turn it off globally in the Output Menu, and set it per profile in the Cab menu. Having PureCab off globally allows you to set it individually per rig. I'd recommend experimenting with it on and off and see if it helps with any of the fizz.
Hope this helps!