Is powerfull Amps only for low sensitivity speakes?

@willemj 
Just crunched the numbers on that. My 30 watt amp is EASILY capable of delivering those kinds of impulses into a 4 ohm load to produce nearly 107dB. So.... What's the need for 500 watts? 

So I guess you started the equation with a lower power requirement for average level, by e.g. using more efficient speakers and/or a small listening room. And 4 Ohm is also pretty low.

+1 - Agree with Trelja and Noromance

From my 35+ years experience in this hobby, power corrupts, and have your ever really heard how loud 1 watt/channel can get?  We only have 1 set of ears and I like mine to last as long as possible.  A lesson I was taught many years ago by a stereo shop owner who was tone deaf but built the most amazing sounding speakers.

@willemj 
I based those numbers on what I'm sitting here dealing with in my condo. 90dB/V/M speakers about 12 feet away with usual boundary and coupling factors. I picked 4 ohm instead of the nominal 8 ohm rating Focal claims just for fun and it probably is more realistic. The F5 transitions out of class A into 4 ohm at 31 watts and easily carries the 24.5V and 6.1 amp to make for 150 watt transients. Looking around here, 12 feet seems a fairly average distance and 90dB sensitivity is pretty average sensitivity. 

 I have heard great sounding systems with both very efficient speakers and not so efficient speakers. But I do feel that with dynamic drivers if you go below around 90 DB you cannot get the really really great sound.  No matter how much power you add you cannot get the macro dynamics.   
 I believe the industry moved more towards less efficient speakers because they are much cheaper to manufacture as you don't need the tight tolerances.   And with the transistor it was easier to get big power so efficiency was not quite as important, or so they thought.