How much amplifier power do you really need?

[RebelMan]

The problem is people believe more power will sound better because some joker says it will and they have no reason to question that. 

That joker would be my ears. And that difference is most noticeable at lower volumes.

I can try to explain (to myself as well as anyone else here) why there is a difference but I'm not sure how accurate that would be. Let's just give it a try:
Say you want to travel on the freeway at 100 kilometres per hour. You probably only 'need' 50 horse power in your car to achieve that. But if you had a nice 6 litre V8 you're going to be doing that 100 kilometres per hour a lot more quietly than with the little 50 horse power car. A more powerful amp might sound more 'relaxed' at any give volume.

I get where you are going but the analogy you are making doesn't quite suit the situation here.  Let assume two similar amplifiers that only differ in total output power, say one at 100W and the other at 200W.  The more powerful amplifier is not going to sound better delivering 20W than the less powerful amplifier would delivering the same 20W.  Likewise, having two similar engines that only differ in total output power, say one at 300HP and the other at 600HP, the bigger engine is not going to drive at 60MPH any differently than a smaller engine will.  It's only at the extremes that more powerful amplifier and bigger engine are going to make a difference.  The question is, how likely are you to drive an amplifier to its extreme?  My lowly D140 can push SPL's to 102dB in my room.  That is incredibly loud.