Hello. I have a marshall jcm 800. I have pedals in both the regular guitar line and the fx loop. When i switch the "active" button on for the fx loop, there's no sound coming through the amp at all. When the fx loop switch is on bypass, the amp works but has terrible growling at mid to higher levels. I know i have all the connections correctly connected, so the connection shouldn't be the problem. I also just replaced all power tubes and preamp tubes but still has that bad growling.

So does anyone have a clue whats wrong with my amp?
heres the symptoms again...
- No sound (not even buzz or static) coming through amp when FX loop switch is active
- Alot of growling through amp when FX loop switch is bypassed.