If I were considering dipoles, such as electrostatics, I would definitely try them along the 15 foot wall, but box speakers have generally sounded best along the 19.5 wall. I think that the sloped ceiling creates an amphitheater effect.
Devore 0/96 and Room Size
I've really enjoyed the 0/96 every time I've heard them and there are a couple of used pairs for sale now. My biggest concern is that my room is 15 feet deep. I believe at least 8 feet is recommended so that the drivers blend and I don't want them closer than 3 or 4 feet from the wall behind, which only leaves 2 or 3 feet behind the listening chair. Would love to hear from owners about their experiences. Thanks.