I own an Acoustic 360 preamp and I am taking the output and putting it through my Sunn head for a power amp. When I turn the power on, I am hearing a single tone and it is not the electronic tuner. I took it apart and there are no loose wires or issues with soldering. The only problem I saw was a very messed up vari-amp knob. It is all corroded but still in place pretty good. Any advice?