If mains noise gets into an audio system, it's because of crappy (eg cheap) design. Any Audio designer should assume that the mains is very, very noisy.

And if it does happen, a simple noise filter should fix it. Which would mean that the amplifier lacks even basic filtering.

There are specs which the electrical supplier must comply with. If you complain of dirty power, the supplier will fit a pen recorder (or modern equivalent) for a week or so to check if it's true.