Tom, the scenario of realistic mains power, combined with realistic transformer and conventional power supplies I investigated about a decade ago, using Spice simulation of the circuitry. Even with still highly ideal parts the impact on the behaviour of the following circuitry was clear - no measurements of the "real world" are needed, even at the 'theoretical' level one can see the issues. At the time I developed a combination of filtering circuitry to deal with it, which I installed on the Philips HT system - the latter had relatively mediocre power supplies, but the added filtering lifted its game to get the good results I posted on. The filtering wasn't perfect, interference still was evident - but it gave me sufficient quality to prove the point.
The headache in all this, always, is the better the playback, the easier it is to hear the presence or absence of the remaining artifacts - leaving a layer of sludge in the sound is sometimes the best way to mask the remainders, it's a form of dithering, effectively.