I think you are flailing for a solution right now and we need to get you back to simple, and straightforward, though that may take some effort on your part.
What are your main speakers? How have you connected them? Is there a high pass filter in place? Have you measured your current state? How large is your room?
How about speaker placement? Try the AM Acoustics room mode simulator first. It will help you see where the lowest modes are. Keep all your speakers and your listening room out of them if possible.