My Carvin R600 Series II head has started to make this noise that sounds like static. It does this even with no input signal, and is independent of where any of the knobs are. It seemed to come and go, but now its there all the time. Once I start playing at volume it gets covered up, but it's annoying and I'm afraid will just get worse. Is what I described indicative of the preamp tube going bad? Or do you guys think it's worse than that?