WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
I construed the initial aim to principally be the ability to make a useful measurement. Even my crappiest cheap DVM happily resolves 1mV (ie. 1mA with 1R on 200mV range), so I have settled on just using 1R for all my output stages.