Just to be clear, if the issue is becuase of "sag" in the delivery of power (which it likely is if it is related to load for the whole service area at different times of the day...) re-wiring will not do anything to help. What MIGHT help address this is a very substantial "power conditioner" -- typically you would probably want something that is capable of not just "bringing up the voltage" for the amp but with enough capacity to also help keep the rest of your electronics from suffering.

I have experinced commercial PA amps doing an "audible resonance" type buzz when the line voltage is signficantly below specification and have used a "variac" which is a specialized kind of transformer that can "dial up" the voltage in such conditions. Unfortunately I don't know if these are really the best solution for a home theatre as they can be rather ugly ...