So where do I go from here.???
It was a Diy amp, so perhaps the builder decided to put in 12 volts to the driver tube. I guess there is a way to check this.
Hey!
OK, so are you not the original owner/builder of these amp. Sound like whoever built these set modified them to take 0528 tubes. Depending on how the heater are wired up 12AT7, 12AX7 and etc can be run at 6.3V in parallel. But do check with people with more experience on tube than me though.
Anyway, if the 0528 tubes sound good to you now. You might want to stick to it and look for some, maybe stock up a few pairs.
By they way, I'm also a Welborne Labs SET amps owner. I have a pair of the Apollo monoblock SET. I'm the original owner bought new as a kit and I built them back in 1999! They just turned 23 years old last month!