So, I have a Fendery head It has a tonestack bypass knob in the circuit. I plug in to either input hi/lo and at times, the volume drops out and the headroom is severely decreased. It is intermittent, no rhyme or reason as far as I can see. Almost like something is touching/arcing somewhere. I hear no "arc" noise though. Just headroom and volume drop by about 20%. No matter how hard I hit it, it does it. I have swapped all tubes in the amp, so I know that it is not a faulty tube, unless I happen to have two tubes that are weak. I dunno. Any thoughts?