100v,115v versus 230v

"Unless a piece of equipment, like a monster power amp, consumes more than 1440 volt-amperes continuous power it’s a violation of US electrical safety code to install a 240V branch circuit and receptacle in a US residential dwelling."

I note that in addition to the voltage also the frequency changes,
230-240 V 50 Hz and 110-120 V 60 Hz.
In my humble opinion, the effect of the different frequency must also be considered,
or at least postulated that this difference is considered negligible.
In a rigorous scientific study it should be proven, not in this case

A transformer designed to work optimally at 50Hz also works at 60 and vice versa, of course, but since these are non-macroscopic listening differences,
a 20% frequency difference may not be so negligible.
Thanks in advance to whoever answers me .