I would start by crossing all of the subs at 80hz. You might consider running just the PSW-150 to get it located in the room (I prefer my PSW-125 on a sidewall firing across the room) then bring the onboard units in your RT-20's back into the mix adjusting their output levels to blend with the external cabinet.

This is just a WAG but my thought is you will want to back off the output levels in the RT'S letting the external sub do most of the heavy lifting.