Is powerfull Amps only for low sensitivity speakes?

@sfroth  good point about inefficient speakers hiding noise. Another thing they hide along with that low noise is micro detail. 

 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.