I am thinking about doing this same thing. My room is (or will be -- house is still under construction), a dedicated home theater. The riser will be about 18' x 6' and 13"+/- tall. I am curious about the mouth of the horn. I see any answer to Jonathanc's question, but I, too, am wondering how mouth placement affects performance. My positioning would, presumably be more like Fun's, with the mouth(s) open to the middle of the room. Does the lack of corner loading negatively affect the performance?
I love Oklahoma Wolf's response curve, and I would give up a redundant body part or two to get similar results.
I built quite a few sealed sub boxes in my younger days (just using the recommended volume by the mfr). I can build the boxes without any problem, but I am completely clueless on the design side. I have zero experience with horns. That being said, I like the incorporation of the subs into the riser for aesthetic reasons, and the horn performance seems ideal for my large-ish room (3000+ cu ft). I would like to build something like this, and you all seem like a wealth of knowledge.