"Can you explain why lowering supply voltage (variac) would burn out resistor in series?" Just thinking there may be a corollary, here: An AC motor, connected to too long of an insufficient gauge wire/cord and the attendant voltage drop, will usually overheat, do to a higher current draw. Will the power transformer/filter caps try to draw higher current, at charge-up, at a much lower voltage than intended? One would think a fuse would go first.
Variac or not?
I acquired a Marantz 510M amp that had not been turned on for about 10 years. Before it went into storage it was in use weekly if not daily as the acquired couples main audio system. So my question is with this amp is it or is it not advisable to use a variac on a non tube amp like the 510M?