Sirquack had an EP500 in his 8,000 cubic foot entertainment room before upgrading to an EP600. According to his posts, the two are very close in performance.
When I first received my 600, I tried many different locations in my 4,000 cubic foot room. And I tried vertical and horizontal. I even tried positioning it outside of my main room at the bottom of the stairs firing up into my main level. I also laid it horizontally right behind my couch. No matter how or where I positioned it, it filled the room with deep, powerful bass.
I'm not sure what you've decided for your room lay-out. I'm hoping that you decided to locate your equipment where you are showing your couch. You don't need to locate it between the two fronts to get good sound. Undistorted bass (unlike the Klipsch) is omni-directional. In my opinion, what would make the most sense for your room is to lay the 600 horizontally below the window. I think this will offer the best performance while giving you the flexibility to locate your A/V equipment between your M80s on/in a low profile cabinet. Also make sure though that the sub is elevated from the floor with spikes or feet to prevent your floor from coloring the sound.
