One of the benefits of the SMS is that you can place one sub up front where you like it and another in your seat for testing. Walking around the walls while looking at the display will reveal your best placement options.

In that huge space I'd go for 2 350v2s rather than one 500. That would work better with your main objective of smoother FR across most seats. I'm currently running with a quad of subs in a 3500cf space, one on each midwall, and FR is about the same now in all seats.

John