Is powerfull Amps only for low sensitivity speakes?

Interesting discussion.  I think at a certain point it just becomes about taste and experience.  Also what are our sources?  I'm a klipsch man myself, and am only interested in a pure analog system; mostly acoustic vinyl, tube pre, low watt tube amps, super high sensitivity horn speakers (Chorus).  I recently upgraded from 150 watt hybrid amp (moscode 300) to 20 watt monoblocks (quicksilver horn monoblocks).  I loved the moscode but there was always a little background hum, the lower watt amps are dead silent, and, in my very subjective opinion, show slightly more detail.  But my system certainly doesn't go as loud as it did, I tradeoff I can live with.  I've heard quad electrostatic speakers that are power hungry and sound incredible, but I decided not to pursue as I'd have to rebuild my whole system around them.  There is also the question of how much electricity you want to be burning.  To me the elegance of big, highly efficient speakers with super low watt amplification is very attractive (I'd love to try a really low watt single ended amp on my Klipsch), but I'm sure the right combination of small, inefficient speakers with mammoth solid state amp can also produce great sound.  In the end it's a matter of aesthetics, musical preferences, and $$.  

This may be interesting: http://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/amplifier-matching-mismatching-and-cli...
The point is that with significant dynamic range the peak power required is much higher than the average. In the quoted example, if you need 1 watt on average, you will need 160 watt to cope with a 22 dB peak above that average.

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.