Lol I too have played gigs where the sound guy is the audience lol. It's happened when I'm still or slightly moving. Seems to be only happening on stage. And it doesn't happen right away. Seems to happen after an hour or so on stage. I'm going to try moving the back monitor and see if it occurs. Thanks!
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?