I got a little behind on this one, sorry. Your 7.3ohm load calculation means your 72VAC was producing around 710watts. Very close. You said your mains voltage was sagging which would probably make up for the rest. Not sure why you are using the variac at this point, you shouldn't need it. (unless you are trying to make up for very poor mains voltage).
If you still have the amp, here's something to try as far as the phasing. Same setup bridged into load but put ch.1 scope probe on input signal. Ch.2 probe first to (+) output, then move it to (-) output. One should be in phase with input, other should be out of phase.