Dual subs, especially if they are positioned on opposite walls, will even out peaks/nulls throughout the room, not just the sweet spot. 4 subs on all four walls is even better.

M80s VP180 4xM22ow 4xM3ic EP600 2xEP350
AnthemAVM60 Outlaw7700 EmoA500 Epson5040UB FluanceRT85