speaker impedance

Well, if we are going to make an argument about distortion, it's good to take an example of a decent linear amp:

8 ohm, all other things equal

but they never, ever are equal

read: impedance curve of speakers... a single, nominal figure means very little...

music is not a handy dandy 1 khz sine wave

Personally never paid attention to ohms or sensitivity until I looked how they sounded. 
 

And remember it’s a nominal ohm rating - hence the importance of having an amp compliment your speaker choice.