A project like yours was the subject of an article in HiFi+. Try checking out issues from Jan-March('09).
Building a Music Room
I am toying with the idea of adding a music room on to my house. It will be for 2 channel only SET amp power and 110db efficent horns without any subs. With the speakers in the conners what are considered to be the best dimensions for such a room? Any advice or ideas will be appreciated!!!