It’s the good old fashioned ear cleaning by a competent ear doctor. If you have never had this done do it now. It’s even a freebie if covered by your medical insurance.
Best tweak
Sometimes the simpleist tweaks are the best ! The best one I've tried lately is to use some rubber door stops to tilt the center speaker down from the top of the monitor towards thr listeners' head. What are some others that have worked for you ?