I'll second the room acoustics. Here is a resource page on our website. It has lots of books and web articles that can help you. It even has a free room simulator. Your 15 x 15 dimensions are difficult, and you will likely want to experiment with an assymetrical placement that can at least reduce the modal problems at the listening position to some degree--or over time you may decide to correct for the modal issues with a paramtric eq such as the Rives PARC.
Acoustics Resource Page
Acoustics Resource Page