I found this answer on another forum, it seems you need to have something plugged into the input of the amp to get an output, even if you are only sending that signal in through the effects return:

Originally Posted By: ..
Eureka! I found the solution!

Plug the TS in the "Input" of the amp, not the "send" of the loop and it works!

Thanks guys for your help


Found it here: https://www.kemper-amps.com/forum/index....HT-40-no-sound/