On/off cycling the worst, in rush current at initial startups hard on tubes, rectifier tubes and/or soft start circuits, rectifier tubes may be sacrificial here. Heat is the enemy of all components, my 845 SET amps, nearly 1000V at plates, cooling fans beneath amp to push heat out, 300B amps much less stressed by the heat. Quality tubes with good reputation for reliability.
Some amps just harder on tubes than others, some manufacturers really push with high plate and/or heater voltages. Do your research on amps, determine whether there is history of failures, this is number one with me. For some years I was having failures with a certain tube, replaced loosely spec'd resistor in heater circuit with much higher spec resistor, no further problems.