1. Build in the wires for 7.1 now...buy speakers later.
2. I have no direct experience with this so I don't know.
3. Since you want a great system, you should buy 80s. They are a glorious speaker that no one should be without.
4. I've heard the 150 (which I own), the 100, and M22s in various configurations. The best I've heard is
M44s.
5. One 600 or dual 500s is what you need. If all you care about is good bass within a limited aread (ie. the sweet spot) get a 600. If you want more even bass throughout a larger area, go with two 500s.
