Fisher SA300 biasing

I would recommend finding an electronics-tech in your area.
I am certainly no expert in re: to tube electronics, but it sounds like an oscilloscope would be the most accurate way to take readings.
Guitar tube amps often have a hum balance pot which is adjusted by ear for lowest level of hum coming through at the output.
My suspicion is that when these amps were made, there were fewer sources offering matched pairs of output tubes-so if one tube in a pair failed, the ac and dc balance would help to compensate for differences in individual tube characteristics.
If you can find an experienced guitar tube amp builder or amp-tech, that might be a good starting point.

Thanks. Since I last posted, I've read up and think a dual trace scope isn't an unreasonable instrument to own and is perhaps the only way to get this amp perfectly aligned. I think Fisher had a precision amp in mind when it was designed, and although other users have been able to "rough them in" at first, they found that using a scope turned out to be the best way to get this amp to realize it's potential. I rather like the idea that Fisher put so many adjustments in there in the first place.