Single vs. Dual Subs

It's common wisdom that dual or multiple subs help smooth out bass response in different spots in the room, but what about for a dedicated listening room with a fixed single listening position? What do two subs add to the music presentation that a single dialed in sub is missing provided that single sub is dialed in perfectly for the listening position?
Caveat: not interested in the SWARM method or multiple subs at the moment, strictly comparing single to dual subs