Does your system occupy a rack or cabinet in between your speakers and, if so, how far away from the front wall are your speakers? Even if you don't normally have domestic permission to move the speakers out as far as 1/4 the distance from the front wall to the rear wall (a usual recommendation), it might be worthwhile to try them there temporarily to see if what you're dealing with is a sonic reflection off of an object located between the speakers.
If the above doesn't apply to you, then also, sometimes, construction variations within the walls of your room can lead to the acoustic center of your room not lining up with the mathematical center of the room (laterally). You may want to try differing sets of speaker positions left to right with respect to the midway point between the side walls...just to see if it has any impact on the problem or if a new lateral position for your speaker pair can be found that fixes the problem. In the end don't be afraid to try experimenting with longer or shorter distances between the speakers as well, but you can start with keeping it the same as it is now, for simplicity's sake.
In any case, this sort of problem is almost always fixable if you stay with it, so keep experimenting. Regards. John