Hi,
I get the same issue with my Kemper rack, I just checked the manual and the entry impédance of the face input is 1MΩ, that will create an antenna with your guitar. On valve amps, it is generally about 10kΩ. I just put a little guitar buffer between my guitar and my Kemper and the buzz disappeared without affecting the sound.
I made it by myself with a simple transistor, some resistances and capacitors:
http://www.learningaboutelectr…sistor-buffer-circuit.php
I hope this will help you to figure it out !