Mains filters, power conditioners, audiophile mains interlinks, supercap-based power supplies:
We can't deny there are some issues with mains power supplies and the effect these have on sound quality.
The mains ac voltage needs to be converted to DC voltage, this conversion is not perfect either and we get problems with rectifier switching noise and hum.
Mains powered devices that are connected together create unwanted ground loops and ground loop interference. This is another problem we get with mains power supplies.
We use battery power supply, it eliminates mains power supply issues and ground loops.