[Grit]

I have owned both a 9BSST and a 6BSST powering 4 ohm speakers and I can tell you first hand that the 6B was smooth sailing and the 9B had difficulty with the load. 

As mentioned by others here  - there is no such thing as too much power. If the price is anywhere close between the amps the speaker is better off with an amp that is cruising to support the load at whatever volume you chose over one that is having difficulty even at low listening levels.

Ditto... went from a 9B-SST2 to a 6B-SST2/4B-SST2. Give your front 3 speakers the power./reserves. they probably make up 80%+ of what you're hearing anyway in a movie.