high voltage's affect on amplifier performance

in much of nassau county, voltage readings have increased during the last 3 years.

at one point i was getting readings as low as 117 volts. this past summer, readings have increased to 126. currently, readings are around 122 to 124 volts.

i have a hypothesis that frequency response is affected by input voltage to a tube amplifier.

is there a scientific principle, or mathematical equation which can support my hypothesis ?

if so, i might want to purchase a variac.

thanks for your help.