My brother's recently acquired 1967 Pro Reverb has a reverb problem. It doesn't work! I've gone through a number of trial and error processes, which include the following: - Replacing the reverb driver and recovery tubes (originals and replacements were both fine) - Checking the tank - Checking the cables - Checking the reverb pot The thing is, I'm able to get the reverb signal to come through the amp when i jiggle or smack the pan. When i turn the reverb pot, i can hear the signal increase/decrease which makes me think the pot is fine. When a guitar is played through the amp, however, I get no reverb effect. Any idea what's going on with this? Should I try a new set of RCA cables?