A single hotplating power tube is more than likely due to a bad coupling capacitor, not a bias problem. The coupling capacitor is leaking some DC to the control grid of the power tube, and reducing the bias of that individual tube. Even if the other coupling caps aren't leaking DC, they're so old their resistance has increased over time, which can dull the sound.

If the amplifier has the stock filter and cathode resistor bypass caps, they also need to be replaced. Replacing the coupling capacitors, filter capacitors, and cathode resistor bypass caps will restore the amp to its original performance. You can also replace the plate resistors in the gain stages and the 3.3 megohm reverb/dry mixer resistor with Kiwame resistors, which will lower the "white noise hiss" the amp makes. It's also a very good idea to replace the main power supply and bias supply rectifier diodes with ultrafast/soft recovery types, and to increase the filtration capacitance on the bias supply significantly (from 50uF to about 250uF).

It all sounds like a lot of work, but it's a very standard "refresh" job for these amps. Any good guitar tech can accomplish it in a couple of hours.

FWIW, my personal favorite power tubes in these amplifiers are Winged C SED 6L6GCs. To my ears, they sound remarkably close to the fat bottle Sylvania STR 386 6L6GC. Electro Harmonix 12AX7s and 12AT7s are fine for the small signal tubes, but the new Mullard and Tung-Sol reissues are real knockouts for just a little more scratch. The Sovtek 12AX7LPS is a mighty fine sounding 12AX7, but it suffers from microphony problems in most combo amps if you play very loud at all.

Thermionic