I have put a posh-looking Belkin unit between my UPS and my modems because I was worried about the possibly non-sine wave character of the UPS’ synthesised mains output. The Belkin ‘AV’ unit was advertised as a mains conditioner as well as a surge protection unit and it’s intended for use with hifi equipment or home cinema kit. So I’m hoping it will ‘purify’ (ie low-pass filter) the mains so as to stop any higher frequencies from getting into the modems’ power supplies. This may well be foolishness on my behalf, as electronics is definitely not my thing. The modems and all other networking equipment is on a mains ring that has no other kit on it, the idea is to keep that mains quiet and also prevent it from being affected if something in another room trips its MCB and causes that mains to be cut. It’s not shared with other rooms and all the other upstairs rooms’ wallsockets are on a separate ring.
So I have made efforts to promote mains cleanliness and keep any possible injected low frequency noise down, which in my imagination might lower error rates or improve SNRM. Given that I have copper lines that are 4.5 miles long the signal that arrives in the house is pretty low amplitude after 64.0 - 65.5 dB of downstream attenuation, so I reason that compared with a received signal that low, even quite low levels of local (in-house) noise may matter. However, those local noise levels would not affect normal people in civilisation whose attenuation is far less, so their received signal is far far stronger, and this includes all FTTC users. So if you’re an FTTC user then I doubt that any of this matters in the slightest.