What is your sound source? Have you tried other sources/inputs?
Have you tried headphones and speakers?
Does it happen at all volume levels?
If power line noise causes a problem it's usually literally noise... Buzz, hum, or whine, etc., in the background. It would be worse (most noticeable) at low volume or with no signal. You won't confuse noise with distortion.
If you're getting audible distortion from some kind of defect (under "normal conditions"... not overdriving your amplifier, etc.) it's usually in only one channel and that can often help to track it down... Switching the left & right connections to the speakers or at other connection points.
I've tried 3 different sources, even using the same tracks for consistency. I've used two different preamps, two amps, and even connected my Ifi Neo streamer to an amp directly. The amp conveniently has gain controls. I swapped speakers and even got desperate and changed speaker cables. None of it made a difference in the noise which is present in both channels. It's definitely not 60 cycle hum, I'm just running out of ideas.
It sounds like distortion. Similar to the sound of an electric guitar with an overdrive pedal. The type of buzzy noise caused by a bad driver but it's not. It's present at all levels but tracks with the volume, not a static background noise. It's worse on poorer quality recordings for some reason but even on newer high quality material it's present, just to a much lesser degree.
The only recent change was new speakers, Tekton Ulfberhts, which I've have about a month. Everything was fine until about a week ago when this noise started out of the blue. At first I thought perhaps a couple of drivers in the speakers failed but I hadn't been pushing them hard and it was both channels. Swapping the speakers confirmed they weren't the culprit.
Having run out of things to try I've ordered a cheap Fosi BT20A. Figured I'd stream Bluetooth from my phone just to eliminate literally everything in the current system except the speakers.