Was having problems with my SVT3pro. When I cranked up the treble and pulled back on the neck pickup I got this insane wobbling whistle. Very loud! When I turned down on the gain, treble or put both pickups back up to maximum it stopped. I assumed it was the tube microphonics since I hadn't changed them since buying it second hand a few months ago. The guy before me had used it for about ten years without changing them. Anyway, I routed my backup Trace Elliot preamp through the power amp stage and it worked fine (I also got a ton more power out of the Ampeg). Yesterday I replaced all the 12AX7s with new Electro Harmonix tubes and the thing is still making the whistle noise. The tweeter is not to blame coz I turned it off and the Trace Elliot allowed me to do all the other tone things without a whistle. The weirdest thing is how the pickups can affect whether it does it or not. The bass is a Marcus Fender with stock single coils. Any ideas??