Are you considering multiple subwoofers, or have you integrated multiple subwoofer in your system? Brent Butterworth goes over the pros and cons of using multiple subwoofers in a home theater.
The two drawbacks with the system Brent used was the lack of individual gain control and individual time delay. If he had these two controls available on each subwoofer they would have made a huge difference in the measured output. Mark Seaton perfectly explains it in the comment section, "You can only get a +6dB summation if the subwoofers are delivering equal output to the listening/measurement position and are in phase. If one sub location delivers much lower level to the listening position, less will be gained."