are you using the powered output into the monitor? Also when doing this do you feel any heat coming off the kemper? Mine would heat up pretty bad when using a passive cab after about an hour into practice at high volumes. If I remember cutting the low frequencies helped stop the weird phaser effect for me. Also I remember it only being certain notes that would cause it. If it is the vibration of components inside then maybe putting a layer of foam underneath it may help
I forgot to ask is the kemper sitting on top of the monitor?
Mine is non powered going xlr left and right to FOH. I place it on top of a chair. It doesn't heat up.