I'm building a house in which the space I intend to use for my HTR will be enclosed on three sides but open at the back. Across the front it'll be about 15' and about 18' down the sides to the back of the rear row of seats where the side walls end and the room opens up. This room cannot be enclosed due to functional requirements. What I'm wondering about is how this will affect the acoustics and what I can do to counteract it. Also, I'm not sure where to mount the rear channels because the back wall is about 18' behind the rear row of seating.