I have a Marshall Origin 20 combo. I have a TC Electronic HOF mini pedal I use for reverb with this amp. I generally run everything right into the front end. Of any amp. My whole life. Always have. I’ve tried loops, but I always tend to think everything sounds better in the front end. Decided to try the loop on the Origin with the HOF mini. It gives me zero reverb. Amp sounds exactly the same. It doesn’t have any effect on it whatsoever... except... It causes feedback. It starts very low, and gets louder and louder until that all you hear. Even with no guitar plugged in. But no reverb... I thought maybe I had the cables backward, even though I checked them many times. So I switched them. It does the exact same thing. Unplugged the pedal, while leaving both cables just hanging out of the loop jacks, and the amp functions perfectly. Pedals in front, amp functions perfectly. Anything in the loop produces zero effect. It just causes feedback. Starts very low, then gets louder and louder until that’s all there is... I welcome your armchair diagnoses.