Yup. You sure would need to know where the ceiling joice are located. I found this part very interesting.
"By using two subwoofers instead of one, and placing them in opposite corners on the same distance from the listener, (it is best done in each corner behind the main speakers) there will be an obvious reduction of the room resonances, while the directly radiated sound pressure will increase. This is because two corners in a room will have opposite phase effects of the fundamental (half wavelength) room resonances acting in this direction.
If we have two subwoofers placed on the floor against the front wall the horizontal standing wave will disappear. If we have two subwoofers in the same horizontal direction instead, but one placed on the floor and one up against the ceiling, the vertical standing wave will disappear.
If we have four subwoofers (down left, up left, down right, up right) then we avoid both the horizontal and the vertical standing waves in those two directions (I then assume mono connected subwoofers). If you want to lessen the action of even higher order multiple standing waves, i.e. full wave resonance, 1.5 wave resonance and others, even more subwoofers may be added to the system. One example of an excellent placement of four subwoofers is an even distribution horizontally behind the main speakers. When the four real subwoofers come to an end, the reflections from the side walls will continue, creating a line source of infinite length.
