This thread has been interesting, and thanks to BE for all the food for thought. I have a few, probably not particularly helpful, comments.
Firstly, with regard to Keiths perennial remarks about something not being properly designed. I realise this is normally trotted out without a moments thought mainly to get a rise out of somebody, but it does seem to me that he may be making assumptions about the design criteria which he might not be entitled to make.
Not being an electronics designer (no, really), Im not qualified to comment, but ISTR John W and others, who are, have said that, for example, power supplies can be susceptible to electrical noise, and/or that if you apply too much regulation to a power supply, sound quality suffers.
What this tells me is that proper design takes into account measures to reduce susceptibility to external influences but also notices if those measures themselves adversely affect the overall performance of the unit in terms of the overall design aims (eg best SQ for the cost), and adjusts the compromises accordingly. Keiths rather blunt assertions make no allowances for such things.
So, if the designer observes that his unit works better with decent mains, or sounds different on different surfaces, he might be able to design-out that susceptibility but if doing so results in a unit that sounds like it is playing underwater, then its perhaps preferable to accept the quirks. Depending on your budget, you may have more or less scope to work on solutions. That's different to 'correctly' and 'incorrectly' designed, which does sound a bit Soviet in its unequivocality. Are we to expect 'an amp designer's response to just criticism' at any point, perhaps?
Just saying.
And my second point is that while BE has been careful to argue that you imagined it is simply a possibility which shouldnt be ignored (on which he is undoubtedly correct), others have taken the its not shown up in the vibration measurements to be tantamount to saying its not been measured, therefore you imagined it.
Lets just say that that conclusion does not follow from the evidence. Unless and until you can dismiss every other possibility, AND can show that the conclusion is one you have evidence to support, it's just a different sort of unsubstantiated supposition. Like, oh I don't know, like the ones about microphony are now slated as being. It is, I think, quite rare for anybody on here to be sufficiently educated and experienced in both electronics AND human perception, so that they would be qualified to make these definitive statements.
In short, if you want to rely on a claim that those reporting changes to the sound have imagined those changes, then under the usual rules were entitled to ask for your evidence and supporting arguments. Otherwise, it is only your opinion, just as I heard it, and cant explain it, but dont think it is all psychoacoustic trickery is only my opinion. So please stop using 'you must have imagined it' as an argument unless you have the knowledge and facts to be sure of your ground. Its only what you ask of others, after all.
Bear in mind also that, certainly the way it is expressed by some,
therefore you imagined it can be a highly pejorative comment. Basically, the way some posters deploy it, they are accusing somebody of hearing things, and we all know that hearing things that arent there is what mad people do. They are, in effect, accusing somebody of being crazy. They never come out and actually accuse people of being mad, but youd have to be pretty dumb not to be able to read between the lines to what they are implying.
Thats the bit that generates the heat in these threads, because the targets of the argument are quite reasonably annoyed at being told they are mental the poster might try to deny that that was what they meant, but its a bit lame. So even if it is the accused who raises the temperature of the debate in retort, the provocation has come from elsewhere (hint: and not from the intellectual or moral high ground).
To return to the OP, Id still like to know what is going on. If it isnt microphony (at least in the terms investigated here, and I did suggest some enquiries which havent taken place and no doubt there are others) then what is it? Psychoacoustic suggestibility is, of course, one possibility, but you wont prove it by a process of elimination. Nor will you eliminate it by making people doubt the evidence of their ears.