Is switching from B&W 805S to B&W CM9 a good idea?

The 805s are very good, better than the Rotel will ever show you. I personally would rather have the 805s than the 804s unless you have a very large room. I would rather add a REL or similar sub than go to 804s. If you really want to improve your sound get one of the new Krell 300 integrated amps, the ones that retail for $2500, I forget the exact number.

The CM9s are very good for the money. Very high value. I use to own the CM7s and the CM9s are better in every way. With that said, they are very different than your 805s. If you love your 805s, you will find the CM9s quite a big adjustment. You should also have a big room for the CM9s or the bass will be overblown. The CM9s are more forward in the treble, and much more dynamic in the bass than the 805s.

I use my system for 75% home theater 25% music. I have 805's for the rears, and an HTM1 nautalis center chanel with the B&W ASW2000 powered sub.