Why do people say low power amps should be good for bookshelf speakers?


I was in a BB Magnolia recently and they had a McIntosh MHA150 integrated headphone amp that can also do 50 watts per channel to speakers. The sales rep said it "should be good for small bookshelves but its not enough power for towers". 
I've never understood this line of reasoning.  Towers are typically more sensitive than bookshelves. Is there an actual reason a small amp like this couldn't do just fine for towers that are equally or more sensitive than similar bookshelves?
roberthz
+1 noromances.  I too think the salesman was making a generalized assumption that smaller bookshelf speakers would be listened to in close proximity and at lower volume levels and would thusly require less power. In some applications, but certainly not all, he would be right.
My Monitor Audio Gold 100’s are power hungry 4 ohm bookshelf speakers with 86 db sensitivity rating.
The only reason might be to eliminate over exuberant use of volume, which could damage speakers, so there can be no come back on the retailer.

Maximum power is often quoted when THD+N reaches 1%. In my mind that's a dreadful figure.

Therefore maximum power is probably best considered at half the advertised figure. Just don't screw the volume up to full.
You are correct there are a lot of big speakers that would thrive on fifty watts per channel and there are a lot of bookshelf speakers that would too because they do not handle much power but everything is about mating the right amp with the right speaker so it is best to try the amp with the speaker that you like and see how the combination works in your room with your music with the rest of your system.