Acoustical treatments will not eliminate the room null. Their purpose is to reduce reflections and echo. Your room is too small to have that problem.
The physics is, bass in any room is strongest when you sit close to a boundary, and it strongest at room junctions (aka corners). Perceived bass intensity decreases as you move from a corner towards the next boundary (up, down, right or left), or from there towards the center of the room. The picture below gives an idea of the situation, the red zone being bass intensity. (The picture isn’t totally accurate, as the front and back walls should also show reduced “red” at the center of the wall, as well as at the ceiling level).
This phenomenon is easily replicated by anyone with a SPL meter set for C weighting, and a broadband pink noise source: The meter reading will drop the further you move away from a corner, with the lowest reading being dead center of the room (i.e. about head level).
Thus, you’ll get the most subwoofer level the closer you sit to a wall. The worst location for the sub will be dead-center on a wall.
Regards,
Wayne A. Pflughaupt