BTW a 6dB/W factor is equivalent to factor of 4 in power. Should we think that there is some magic factor that makes high power amplifiers sound less good than their smaller cousins?
no 'magic' factor involved.
and the idea that high powered amps sound less good than less powerful amps has some basis....although the reasons that some share that perspective are complicated.
i think that within the same designer's products more powerful = more expensive = better performance. if there are different type circuits from the same designer often the more expensive, less powerful amp, sounds generally better. Lamm is a good example of this....even though you could make a case that the amps sound different.
in fact, i think price is a much larger factor (but certainly not any assurance) on amplifier sound quality than power. and when looking at hard to drive speakers it's more that some amplifier choices are taken off the table that is the issue.
i think many times these perceptions have to do with what people hear at brick and morter dealers or at shows. where big hard to drive speakers are paired with mega amps, and there are simply so many moving parts to those set-ups that they rarely connect to the listener in ways more modestly power amps and more benign speakers can connect when properly set up.....and those systems maybe fit the space better too.
i think that amps with simpler circuits and the best quality build and pieces used sound the best. mostly those minimalist amps are modestly powered, but not always.
there are quite a few very good sounding mega powered amps, but also plenty of decent sounding mega powered amps which don't sound their best when they are really pushed. so a speaker which is a tough load does demand not only lots of power, but also a very 'happy' amp at that high power to succeed. so that tough load speaker that is ultra detailed and knife edged does end up getting extreme reactions since the balance is delicate.