Is your Kemper new?
If it isn't maybe you need to Initiate Globals in the System menu. You might have some configuration creating that harsh sound.
Other possible reason is if you have an audio interface connected to the Kemper, in some cases you can hear twice the sound thru the headphones because you hear the original sound and also the return sound coming back to the Kemper from the monitor outs of the interface.
This would also be corrected when doing an Init Globals in System so I would try that first.