I recently purchased a STL 900 (about a month ago). I've gigged with it a couple of times with no problems. Tonight however, sitting here in my home I turned it on to get some practice in and I hear this fairly loud "noisey" well........... NOISE coming out of my GB 1288 quad. When I increase the volume the noise gets louder. Almost sounds like a scratchy pot but it's a constant sound. My question is could this be a tube thing and if so how do I know which one? Thanks!!!!!