OK, I think we're closing in on it. The flanking subs are a visual problem. Two solutions come to mind. First, maybe I might be able to build a two chamber corner box and essentially stack the speaker on top of the sub and make them appear as one. Might not be sufficient volume to do this, though.
Second, I think I read somewhere here that using corner horn would enable me to eliminate the flanking subs. I'd just use one sub at the back of the room. Is 18" the minimum dimension for the front baffle on your horns?