If you're using a pair of subs, you might get an oscilloscope and see if the signals sent to the two are phased. I used to enjoy using the left and right channels to generate Lissajou patterns and I was surprised how many recordings had bass with content phased more than 90o between channels. If your signals are already phased 135o then the distances to create a 180o shift are smaller. And in a small space, two sound sources would be tightly coupled and highly combinative.

Wayne