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.
you will soon learn that increasing the B+ supply increased the cathode currents and so increased the gains...
what then is your limitation? the plate dissipation ratings, make sure you do not approach it, small signals use a fraction of the plate dissipation ratings when considering the tube operating point.....
When gm and RP are measured, electrical factors affect the result and considerable variance is observed. OTOH, geometry is the principal factor that affects μ and (no surprise) the value observed is sensibly constant.