I was asked about how the wattage of my own KT-45 was calculated. This is the method I use on all amps:
1000 hz tone into an 8 ohm load resistor with a voltmeter and scope connected to the resistor .... raise the level until clipping just occurs. Tone controls set to the most clean waveform at 400 and 1000 hz. My load resistor is a 250 watt load resistor.
KT 45 = 22.8 volts into 8 ohms = 64.98 watts at the onset of clip. If you have an HP distortion analyzer you look for 5% or 10% (depending on whose spec you want to use) distortion level and measure the voltage and apply Ohm's law to calculate wattage.
Ever test with Tube Rectifier? Can we assume a tube will be approx that same?
My results on the KT45 were always with the tube rectifier. With a solid state rectifier, if all is the same ... output tubes and phase inverter, you may see a rise of 3-5 or so watts in some cases over a production rectifier as most of them are on the weak side.