Your situation sounds similar to mine. After 4 months of reading online, I went with an M80 setup. My house is also 1800 sq feet and my living room is like yours except carpeted and one window and vaulted ceiling.
I drove my setup for the first time yesterday with an LMC/LPA combo. I can say it was loud and very clean. I am still playing with the settings on the pre/pro but so far I am totally impressed. One thing I really enjoyed was hearing DD 5.1 effects just watching Discovery HD.
I was able to drive the speakers to insane SPL (hearing was distorting) for about an hour before the thermal kicked in on the amp. The only distortion I noticed was during an explosion scene, it seemed the woofers were overloading. I quickly turned it down. After some more playing around I think this is more related to the very high SPLs and my crossover settings are not correct yet for HT.
Anyway, I am happy with the setup so far and will be doing some more tweaking when I get home from work.
