Bass is omnidirectional unless the sound source is very large. The deal with room modes is interference between a reflected wave and the source, or between two reflected waves. It makes pockets of high and low energy throughout the room at frequencies below about 300Hz.When you use multiple strategically-placed subs, you can fill in the holes. A notch formed by interference between one sub and a boundary's reflection is energized by a different sub that doesn't have destructive interference at that particular location.