So long as Andy is working at a high enough frequency to neglect having finite transformer inductance, he can play the game backwards. Take the impedance of his dummy load, multiply by the OP transformer turns ratio squared and draw that load line across the valve's anode characteristic curves. Find the operating Va and Ia quiescent point of the valve and plot it 3V RMS is +/- 4.24V at the dummy load, so multiply that by the transfotmer turns ratio, and mark out +/- this voltage either side of the operating point. He should then be able to see where the valve is swinging with respect to compression or cutoff.
As you said, running to the region where the curves relate to grid above 0c Vgk will guarantee the onset of grid current and a dramatic drop in the Z presented to the previous stage.
You can tell a lot from seeing a load line plot. If you do start running with inadequate inductance, or with an inductive load resistor, the load line shows hysteresis and opens out from a line into an ellipse which makes life a lot more complicated. You have to do it for transmitters to see the effect of mismatch on output power capability. Audio is so much easier!
If the stage hasn't been carefully designed and a load-line fitted to the valve's HT voltage and current capabilities - and then created in realiy by the right transformer turns ratio, then the available output power is reduced from what it could have been either by cutoff at one end or saturation at the other. The right turns ratio and load meets the onset of both limitations at once, and is a best-fit into the available operating area.
Once a real-world loudspeaker is used on the output, with its impedance varying all over the shop, and a few resonances thrown in for good measure, the operating load seen by the anode goes all over the place too and the available power gets severely curtailed as the load line swings around.
Negative feedback doesn't just flatten the gain of the amplifier, it modifies the amplifier's output impedance and it acts to curtail the influence of speaker impedance on the amplifier.
Loudspeaker designers have noticed the trend to transistor amps, with relatively huge amounts of negative feedback and now design their speakers and crossovers assuming that they are driven from an almost perfect voltage source. Drive them with a valve amp, and they are significantly away from their design conditions. The analysis of this becomes very complicated, but it's needed in order to know what's going on when the two things are connected together.
Alternatively you just connect the two things together and measure voltages/currents over a sweep across the audio band.
It's why I laugh when I come across the attitudes that transistors and feedback are evil
In the background of all this, speaker people changed their definition of speaker sensitivity from dB SPL at 1 Watt, to dB SPL at a defined drive VOLTAGE. It got everyone out of the quandry of having to provide a non-flat drive to put a defined power into a very non-flat impedance, and it recognised the dominance of constant-voltage drive from feedback-dominated amplifiers.
Too long, and too deep, maybe, but it's what's going on. Ironically, the simpler-looking the circuit diagram, the more complicated it gets
The universe has a sense of humour.
David