This Markbass F500 head has been acting up recently in a particular way. Wondering if anyone has any ideas about what might be wrong with it. it basically works perfectly in every way, except that there is what sounds like constant crackling/static noise in the background. The tone shaping and filters seem to affect the quality of the static so it seems to me like it's being introduced before the tone stack. the volume of the crackling is tied to the master volume but not the input gain, i.e. the static volume doest change with the input gain knob. It almost sounds like a scratchy pot or a bad cable but clearly is coming from inside the amp. Because it's an older unit and I'm not even sure about serviceability of these, I figured I would just throw this up on talk bass to see if anyone more savvy than me might know what to do next with this. thx.