From the picture it looks like your old mains are Bose 501s? If so, I think even the M50s would sound better
I would seriously go with the 2xM80 + VP150 + 2xQS8 for now. The M80s are really good down to about 30Hz or so. Without the sub, you'd be losing the 20-30Hz "kick". For music it's not so much of a loss, but you would miss it in movies. If you feel down the road that you need more bass, you can always add the sub. Trading up mains is more of a hassle.
I bought 2 EP500s last month, and I'm still tweaking things. But in my theater (18.5'x16'x8.5'), I only have them set at the 6:30pm position and I'm peaking at 82db in the 20-30Hz range! If I were to do it over, I may have gone with 1 EP600 instead.
