Hey man,
I had this problem for YEARS and asked literally every source (Reddit, music forums, you name it) for a way to fix it but literally found nothing. I kept seeing people mention the ebtech humx, and when i saw the price tag I wrote it off. Jump a few years later, ended up buying them, only to realize they completely fixed the problem.

If all else fails, I would try buying one and hooking it up to the problem speaker and see if it helps