Someone else may be able to give you a better idea, but generally you want to use the impedance that, when the tube is idle, shows a voltage of 75% or less of what the maximum plate voltage should be. More voltage at the plate will give you more wattage at the speaker, so go with what your power supply gives you. The higher the DC resistance used, the lower the wattage will tend to be at the speaker. However, there is another thing to consider. The higher the impedance, the better the bass response (to a degree). Choose an impedance that yields good power and good bass response. Increasing the impedance for bass response too much may decrease available power and cause the bass to distort. So select an impedance that runs the tube (no signal) at about 75% below maximum rating (if this is possible....lower if not), and gives the best bass response.
Thomas Dermody