I was playing an Ampeg SVT III-Pro that i bought off ebay and is about 10 years old. When I play, no matter which way I turn the tube gain knob, the tube sound remains the same, and I can not get a clean sound at all. I was hoping it might be the preamp tubes. Would they sound fuzzy all the time if they were going, and not get as gainy as possible when cranked? This problem exists no matter which way I turn the gain and master. I am using a passive bass, so its not the bass and I do not use any effect pedals. Any advice ?