As near as I can determine the WZ68 sag resistor is about 38 ohms (17V drop @ 450mA). The resistor's purpose is to simulate the tube rectifier voltage drop as current increases. The would modulate the voltage supply slightly but the ST-120 has so much capacitance that the effect is probably minimized. But would like to consider elimination of this waste of power and the heat it generates. If, at idle, the ST-120 high voltage generates about 230mA current (guesstimate) so the 38 ohm sag resistor dissipates 2W but even more playing music.
And going one step further has anyone used solid state diode rectification only (no resistor or thermistor limiter)? I fully understand the purpose of the thermistor is to limit inrush current. Removal could be hard on the tubes at start up so just asking. Thermistors also get warm to hot in operation and another source of wasted power but not as much as the sag resistor. And yes a relay board or standby switch would be critical to hold off high voltage until the tubes sufficiently warmed. And with the changes assume the mild bump in high voltage at the first couple of filter caps is OK. I could live with the WZ68 no problem but the device gets quite warm in operation.