About a week ago I completely lost gain and volume in the lead channel of my 100w Legacy head. The clean channel is fine, but the lead channel seems to amplify a bit of the pickstroke and nothing else - no volume, and no change no matter how I tweak with the knob.

So I bought a couple of 12AX7s to replace the preamp tubes I thought might be faulty, but no dice. Same issue, no change. I was curious, so I moved all the preamp tubes down the line one by one trying to find if one was blown, but the problem didn't change or move around at all.

Because the clean channel works just as well as ever, I'm assuming it's not a power tube issue. Does anyone have any idea what this could be, before I blow a wad of cash on a techie diagnosis?
It could be the preamp tubes for the Overdrive channel I suppose.

Roger Waters - 12th May!