I did the same, stacked them yesterday and holy CRAP movies were incredible. U571, Ep2 when the ship blew up in the beginning, holy moley - good stuff. BUUUUT ... I got a nasty null in the mid 30's (about -25db) that I couldn't shake ... so, I scrapped the stacked idea and went back to front center and rear center arrangement. That's giving me the best response for my room. I think I'm done with placements and EQ and going to stick with this. I'm not sure how much more my back can take this! Here's a graph after BFD eq with room curve (NO smoothing). About 3500cft connected to a great room ... almost 10000cft total.
Btw, how the HELL do I show the image in the body here instead of an attachment? Can I use Flickr?