Almost identical room here. I started with M80's and a 2-channel integrated amp. 50/50 listening between music & movies. I was absolutely thrilled. Then about a year later added a 5.1 receiver and a second hand sub. When watching movies, I ran it in "ghost centre" mode. It worked remarkably well. Eventually was able to buy the remaining speakers in my sig.
Buy the 80's, move the Def's to the back for surround duty, and run the Onkyo in "ghost" or "phantom" centre mode. You won't be dissapointed.
Epic 80/600 + M3's + M3 Algonquins + M2 Computer + EP125
I think I'm developing an addiction.