Ok, breaking it down step by step;
The principle at play is Selective Mode Cancelation
1.) A room's resonant modal behavior dominates the frequency response and the sound we experience in the low frequency range.
2.) There are 3 primary types of room modes, Axial, Tangential and Oblique. For our purposes here, the dominant resonances are axial (only involving two surfaces or boundaries).
3.) The distances between a room's boundaries (front-to-back, side-to-side, floor-to-ceiling) determine the resonant behavior in that axis.
4.) A subwoofer placed up against a boundary is located at a pressure maximum, thus maximum coupling or excitation to all modes in that axis.
5.) Leave that sub there, and place a second sub on the opposite wall boundary and you've canceled the odd order modes along that axis of the room due to acoustic polarity.
6.) Now, if we move each of these subs off their respective boundary, and to approx. the 1/4 points in that axis, we address the remaining problematic mode.
7.) By bringing those two subs into the 1/4 point, we're placing them at the pressure minimum, thus that mode is not energized.
We can utilize this approach to address a room's most problematic issues. If the front to back consistency is what one is most concerned with, easily it can be addressed. Side to side across a wide seating area, etc., any room axis, or even all three axis if need be (four subs placed at 1/4 points). Until one measures, you really don't know for sure. There's significance variance in boundary stiffness, and this can throw things off, so again measurements trump all. Soft, slightly moveable walls/boundaries are more ideal, stiff concrete like walls heighten problematic issues sharpening modal Q.
A careful multi-sub approach, is a fantastic first step toward great bass. Multiples combined with as much bass trapping as one can aesthetically handle, can easily yield superior results. Global EQ for the sub system can knock down any remaining issues, and if need be individual time delay of as many subs as possible can further smooth the complex summation of the sources.
Hope this is helpful, certainly don't want to muddy the waters so I'd be glad to clarify specific points.
Flat, Deep, Clean, Linear, and Loud
Active 16.8kw, 7.3 system
(3)Seaton Cat12C up front, (4)QSC K8 sides/rears
(2)Seaton SubM-HP, (4)18" IB