I use a similar method as suggested by Polk432, except that I align the subs (2 15" sealed), facing each other from the outsides of the speakers. Basically, you will aim the subs towards each other across the sides and slightly to the rear of each main speakers. I use a Velodyne SMS-1 sub controller to address any issues. This could also aide in mitigating phase issues.
I have used this process for years with satisfying results. Also, the sound stage seems to open up greatly using this method. Observe that a cutoff below 60hz also removes localization issues associated with bass.
I have used this process for years with satisfying results. Also, the sound stage seems to open up greatly using this method. Observe that a cutoff below 60hz also removes localization issues associated with bass.