I’d first rule out you don’t have an equipment problem if you haven’t already done so. If you switch the speakers and the problem switches sides then there’s likely a speaker problem. Not sure if it works the same with electronics (maybe someone else can chime in here), but I’d think if you swap the R/L inputs on your amp or pre and the issue doesn’t switch sides that might indicate an imbalance there. I tend to doubt that’s the issue, but it’s good to rule it out before trying other things. You should also get your hearing checked to rule that out as well.
As others mentioned this could well be a room issue, and if both equipment and hearing issues are ruled out I’d try both absorptive and diffusive sound treatments as sometimes one works better than the other depending on your room and personal preferences. I’m thinking a half-round absorptive column against the wall between the speakers might be one of the first things I’d try along with some form of treatment at first reflection points, but that’s just a rough guess and unfortunately every room is different and thus usually requires some experimentation to ultimately find the right solution — well worth it in the long run though. I’d start by talking to at least a couple established room treatment companies (GIK, etc.) and describe your issue and your room and see what they say and recommend. Hope this helps, and best of luck in solving this!