I found schematic of 510M and note that "Resistor R3 must be shorted with a jumper before autotransformer is used".
In normal operation there is a time constant of about 0.3s to activate relay that shorts this resistor, but perhaps at low supply voltages relay might not turn at all. This resistor is 10 ohm in 110V version of this amp. It is hard to imagine how this resistor can burn at idle power. Assuming about 50W of idle power supply current will be about 0.5A resulting in 2.5W dissipation on this 10W resistor, but I won’t argue with manufacturer - they know better, and shorting large resistor is easy to do.
In normal operation there is a time constant of about 0.3s to activate relay that shorts this resistor, but perhaps at low supply voltages relay might not turn at all. This resistor is 10 ohm in 110V version of this amp. It is hard to imagine how this resistor can burn at idle power. Assuming about 50W of idle power supply current will be about 0.5A resulting in 2.5W dissipation on this 10W resistor, but I won’t argue with manufacturer - they know better, and shorting large resistor is easy to do.