Hi Chris,
I can manage to be a lot wrong, it happens often !
In the case of a valve amplifier, the output impedance of the valve will be very high so if this is matched to the speaker, the anode voltage will vary a lot but there will be little change in current.
The power output will be maximised at this volume control setting but will be much lower than that possible. The voltage/current (load) line is set to get the right swing of voltage and current. The load line resistance is roughly the maximum voltage/maximum current.
The maximum power is when the source and the load impeances are the same but there are limitations which normally predominate. In a radio receiver, each stage may be matched to get the maximum gain but this cannot be done when significant power is involved. Then, the load impedance is higher than the source impedance to get most of the power delivered to the load.
Sorry about rambling on, I have not woken up properly yet so I may be wrong !
Duke