Who knows
First, you must make sure the speakers are operating in phase. Sit equidistant, only hook up one speaker with some thumping bass line. Then, add the second speaker and listen to assure they are additive.
Second, SBIR is an issue that can easily can destructive interference from the adjacent walls, typically the wall behind your speaker. This energy comes off the wall and can easily be destructive acoustically and null some energy in the power, or impact range of the bass. So experiment with placement away from wall, or back toward the wall, just experiment.
Listening position, experiment with your seated position, perhaps room acoustics is creating modal issues, again destructively nulling LF energy at the LP. Moving backward toward the rear wall may improve bass,...at the expense of other issues, however,...again, ...just experiment
This is likely what's going on; Poor response in the time domain. Oftentimes, excessive ringing, or LF energy sustaining too long, and this provides for ill defined bass. Bass traps are one remedy, however there are others. Poor response in the time domain is prevalent in many systems.
Read
this , it's very good and relevant.
Here is a solid article about bass traps from Ethan Winer,...an AVS contributor.
I think you may find Floyd Tooles Sound Reproduction book interesting, it's superb. Everything regarding loudspeakers, and the home is within.
Good luck
