You don't mention what speakers you have. Any reason?
Anyhow, try to be sure there is a significant difference between each speaker and the front and side walls. The same distance will give an unsmooth bass response. A difference of 40% is recommended. Also, you can move them closer or farther away from the walls to get more or less bass. I was just reading Avalon's speaker placement recommendations, and they recommend the following relative ratio from wall to speaker to speaker to wall: 4-10-4. These formulae may not all work together, but they are things to try.
Anyhow, try to be sure there is a significant difference between each speaker and the front and side walls. The same distance will give an unsmooth bass response. A difference of 40% is recommended. Also, you can move them closer or farther away from the walls to get more or less bass. I was just reading Avalon's speaker placement recommendations, and they recommend the following relative ratio from wall to speaker to speaker to wall: 4-10-4. These formulae may not all work together, but they are things to try.