No, you can't put an m80 on top of the 600, there is a slant to the cabinet on the 600.
I have my m80's about 6-8" from the back wall to allow the low frequencies to filter into the room from the port.
Also, not sure about your understanding of having the center come from the center of the screen, first time I've ever heard that one. Your eyes should be at the bottom or 1/6th up from the bottom of the screen for the best viewing comfort when seated. Your tweeters for L/C/R should be at ear height when seated. This is for music AND movies it makes no difference. Now some people mount their centers above or below the screen BUT angle them up/down towards the listeners ears. This is because they have no other options.
