Variac or not?

I would power it at about 70VAC for one day.  Electrolyte in electrolytic capacitors eats up dielectric (aluminum oxide) lowering breakdown voltage. Presence of voltage rebuilds this layer.  10 years is about the time I would start worrying.

Can you explain why lowering supply voltage (variac) would burn out resistor in series?

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.

@rodman99999 SMPS would do that - draw more current at the lower voltage, but 510M has linear power supply. Lower supply voltage shouldn’t result in higher supply current. The issue is relay shorting this resistor, that might not activate at the lower voltage (poor design?). I still don’t see how this resistor can be damaged, unless they assume full operation (2x250W) at the lower supply voltage (not desired for conditioning of capacitors).  At idle power it should be OK, but when they say "short" I would short, just in case.