The data given in the datasheets for various lamps were made dozens of years ago. Then there were no oscilloscopes and other accurate measuring techniques. Now we have the opportunity to analyze amplifying cascades and to select modes in which any lamp can fully realize its potential. I mean that at all sound frequencies the lamp should have the same gain, as well as the maximum possible signal of a sinusoidal and rectangular shape without distortion. I think that in such experiments it is also necessary to take into account not only the load resistance, but also its capacity.
Measurement of tubes: 12AT7
The measurement of high-power radio tubes must be done with a complex load, as stated in the state standard for amplifiers. Scheme 1 in this state standard.