Very interesting indeed.
I'd like to add something from my own practical experience. If you are not hard of hearing, are in a "normal" sized, "quiet" room, sit a "reasonable" distance away from your speakers and have relatively efficient speakers like the M80s, a 90W/channel amp should never cause you any problems.
Take my case. I'm in a 4,000 cubic foot, relatively live room (with considerable leakage due to my staircase) that has an ambient noise level of about 50dB with the lights off and 60dB with the lights on. I sit 8 feet away from the fronts. My right ear is not as sensitive as my left (but I don't know by how much). When listening to music, my receiver typically pumps a quarter to a half watt into my M80s. This gives my amp headroom of 22dB to 25dB which is 10dB to 15dB more than recommended.
So let's say you are in a room of similar size, have some hearing deficiency like me but have significantly more furnishings. Let's be generous and say that you will use 4 to 8 times more power than me (1W to 2W). That still gives your amp 16dB to 19dB of headroom!
