Two threads worth a perusal if you haven’t already:
It can be very helpful to get a copy of REW software (free) and a measuring microphone (the one REW recommends cost around £110-120 last time I looked, half to three quarters secondhand and easily sold on if not required anymore), to find out what is going on in the room, and explore the effect of different positioning. Don’t forget that the listening position is every bit as important as the speaker positions. Bear in mind that sometimes unconventional layouts work, especially in awkward rooms, e.g. diagonal placement.
If playing with positioning doesn’t resolve your issues, GIK Acoustics, who make treatment products, can analyse REW files and even recommend what measurements to take, and from that recommend solutions, though of course aimed at selling their products.