I have the UMC404HD
XLR's from back of Kemper go to Front side channel 1 and 2.
Monitors are connected like in your picture to main out A and B jack
Mix is set to 12 'o clock (middle between IN and PB) to hear both computer audio (YT) and kemper
Yes! (Thank you) I got it working that way! 😁
Couldn't get it working because of the 'mix' knob being all the way up.