Indeed, the voltage is not important to most ss amps. however, the noise and other things that challenge your amp to produce quality sound also are likely increased with poor voltage supply.
many voltage meters can have a zero offset, but if he is using the same meter for all measurements, he observed spread of 13 volts is probably real.
Tube amps can benefit from constant voltage, some more than others. My power regenerator provides 119.9 volts 24/7/365 so I never have to think about it.
Jerry