Is DBA/SBA always the best solution?
DBA - double bass array - is a way to fix all bass problems, provided the room is rectangular and you can have bass units distributed across the whole front wall surface (DBA and SBA) and rear wall (DBA).
SBA is similar from an acoustic point of view, but instead of having active subwoofers on the rear wall to absorb the wave, you use acoustic absorption.
The model is simple - all those sub units create a plane wave front, which is then absorbed at the rear wall - this creates a plane wave travelling down the room, all modes and resonances from room reflections are now removed.
In most practical installations, there will be significant compromises - not enough sub units to create a perfect wave front, not prefect absorption on rear wall, side walls not perfectly rigid.
But - it will work very well, if your room can accommodate the proper installation.
Additional problems are that the large amount of wall surface covered with sub units can prevent optimal acoustic treatment.
Question on a forum - A room with 4x sub units (each 18" sealed subwoofers), room quite long, rectangular:
In this room, I would try all 4x subwoofers on front wall, and absorption on rear wall.
Math/model for DBA is very simple - for the perfect situation - and you get perfect frequency response, and a sound field where direction (pv) is predictable and similar to free-space at very large distance, where the wavefront is a plane wave. The problem is, practical rooms do not have rigid wall surfaces and 2x units on one wall is not enough to produce a sufficiently plane wave. And as you approach 100hz and so, the deviation from the model becomes so large, it will be very difficult to tune the system so you avoid hearing the rear sub units.
With 4x and individual processing, you can tune the system using other approaches - experimenting with delay on the rear units are crucial here. the goal should be to achieve a decent frequency response AND timing and preserved sound field intensity/velocity. This can usually be achieved in most rooms with 4x, it does not require a DBA/SBA approach, but it it will require some work on calibration.