[turretslug]

I may be speaking out of turn here, and certainly out of my depth, but does the design/operation principle of these things make assumptions about the input mains source, i.e. a "low" (as in neglectable) source impedance whereas the variac output impedance is non-negligible, so as to upset the transformer's output impedance, or alternatively perhaps to de-tune it?

Clutching at straws, but intrigued as to mechanisms that might impair effective operation!