One thing that isn't often considered is fluctuations is mains supply - forget quality, I'm talking quantity.
We're having weird issues with odd spikes in our mains usage (most days around 12kWh, but odd spikes up towards 17 or 18kWh, usually with nothing to explain the extra usage).
Anyway, I bought a mains tester/meter from Maplin a few months ago, and I've found that in the evening our mains voltage creeps up from a daytime average of around 232-234 volts, to 242-246 volts. I don't know enough about the workings of the mains stages in our equipment so can't comment on how likely this is, but I wonder if an increase in voltage makes the life of these parts a little easier. If the extra volts find their way through to the internal components, it may be a little like overclocking a PC - or adding (mili)volts to aid stability.
In addition my findings, all the sockets on the spur that supplies the kettle show a drop of around 4-5 volts when the kettle is turned on. This part way negates any benefit offered by the night-time increase in volts, but obviously only whilst the kettle is on. Sockets on other spurs show a negligible drop in volts. This has made me wonder what other appliances may have an influence. If your gear is plugged into a socket that's back to back with one in the kitchen, then any activity by the fridge/freezer etc might make a difference, if they share the same spur. Adding stuff from the shops one night makes the freezer work that evening, when it needs little work if the door hasn't been opened all day - could it (or something else) have been active on the first night, but not on the second?
Before the nay-sayers (you know the ones, the anti-mains cable gang) pipe up, the mains meter I use has been checked for accuracy by comparing with TWO accurate multimeters at mains voltage.
My cousin has a 40-odd inch Samsung Plasma, and the picture quality is awful during the day, but picks up at night. All evening the lights in the house brighten up, then dim down, then brighten up again, and so on. The improvement in picture quality coincides with the lights stopping their fluctuation dance. You can't kid me that mains fluctuations don't affect output quality. The house probably needs a re-wire, as the fuse board was changed a few years ago from the original 60's fuse-wire board. Without the expense of a re-wire, the second best option is to try to filter out the mains anomalies introduced by the house. I plan to try my Tacima mains filter at my cousins, and see what (if any) improvement it makes. That will settle the mains filter argument for me, once and for all.