sorry about that. I compressed the images so I could upload them direct to the forum instead of linking to them. should be easier to see now. I will clean the pots today and retry, thank you. you're talking about the two bias pots for the left and right pair of power tubes correct?For some reason I cannot mag up your pictures, even so I suggest you reflow the preamp tube sockets.
We may be getting close to your problem.
I do not think you need the bias board mod, it is more likely to be one of your bias pots being faulty. Clean them both with contact cleaner and apply some Deoxit gold to lube and clean them, then retry. If it does not work they may need replacing.