I have an older 2 channel Triple Rectifier with a pretty messed up fx loop. Whenever I turn it on, the amp becomes very week and shrill sounding and even loses some volume. If I turn the FX master knob, the amp makes a loud pop noise.

has anyone had this problem before? Know what it might be? What I can do ?

My 2 Channel Dual Rev G did that (Used) when I first got it. Turned out the Send pot was bad for the loop.