@nickrobotron what was the bias reading the last time you checked, and how long ago was that?
Hey, it's a tube amp. Output tubes wear out and get replaced. I run KT120s in my tube monoblocks, and the new pricing is high right now, and for the past two years. If you are not aware, Sovtek - New Sensor is in Saratov, Russia. There are bigger problems going on in the world right now, that's why... supply is short now. None of my regular suppliers have them. Yes, some people selling them higher on eBay.
Unless you know the age and hours on them, start with buying a pair and recheck bias closely on the others, replace them too if necessary. Or, ask around and find a local tech, they can replace a bad resistor. Worth fixing the amp, and better for resale working than broken if you decide to move it down the road. Good Luck.