Hi guys! I'm a longtime lurker on this board and I need a little advice. I was testing out a used SVT 3 Pro yesterday and I was pretty happy with it except when I turned the "tube gain" knob it made bad scratchy noises. Also, it didn't actually change the tone once I got it set to a clean sound. The amp had a nice growl in the upper registers that didn't change whether I was all the way up, in the middle or down, so I felt like the tube gain wasn't changing anyway. This is not just a scratchy pot--I know that when I see it. Something isn't right about it. The guy is offering it pretty cheap ($500) so I am not opposed to anything that requires a simple fix like replacing the tubes. Any ideas on what it could be?