Ok. Since you have not yet installed any inside wall coverings, consider furring out 3 of them so that none of the interior wall surfaces are parallel. Still want them planar and flat, that just looks better, but making the room just a couple inches out of square on the inside. Same with the ceiling ridge if possible. Not enough to be noticable except upon close inspection, and extertior walls should still be square and plumb. It's a bit extra work, but will help prevent standing waves in the listening room.