You might want to buy a copy of F. Alton Everest's book on recording studio design (sorry I don't remember the exact title). It's pretty inexpensive and gets into the dynamics of the total room. Optimum speaker placement in a rectangular room depends on a number of factors including 1)wall, ceiling and floor composition, 2) room dimensions and 3)listener position (plus speaker design, furnishings, etc.). I studied the heck out of it since I'm building a new house around a sound room (sounds goofy, but ...). The sites referenced above provide great food for thought and background. If you really want to dig into the factors around this, you might want to read Everest. There are a number of formulas in there that cover the topic. Trust your ears and have fun experimenting. Cheers.