From your other thread you say you have the speakers against the wall on the longest width, 4m wide, and the seat is against the opposite wall 3m away and that the left hand speaker appears to be the worst. Can you, for clarification purposes, move the left speaker temporarily and see if the response improves?
Having speakers and seating at the boundaries is less than ideal for bass response and if you are unable to alter it then, apart from some absorber panels, there is not much that can be done.
Good advice above. I'll also add that if it's different in one speaker than the other, then something is asymmetric in the room. Find that and you're on your way to finding the problem. A top view sketch would help considerably.