Multiple subwoofers cancel the room modes caused by one subwoofer.

Evidence

Having two low-frequency speakers allows one to place them in a way that deals with the inevitable modal issues that happen in real rooms at low frequencies. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2226

Categorization

• XREF: Connects to any prior chapter material on room modes and standing waves; this is the practical payoff of that theory.