Hi JMH.

Q1: NO.

Q2: NO.

The higher the sensitivity the better.

A full range speaker should not be adversely
affected if driven by a low powered head unit.

Only for subwoofers could this cause problems.
Underpowering a subwoofer is as bad as
overpowering it.

This doesn't mean that you should get 300w
speakers.
But getting 50w speakers should cause no
problems.
Just invest in good quality speakers(MB quart,rainbow,
boston accoustics and the like) and you'll hear a clear
difference over the stock speakers.
So 90dB and 10-50W should be good.


Louis.