When I said centre, it's not exactly center, but at the listening position, about 40% of the length of the room. I'd like not to move the setup of the room a lot cause it's a pain, and moving SM9 alone is nearly impossible...
Placing treatment behind speakers will act on modes and SBIR. It should be the best improvement.
It's a totally non treated room for the moment. There's a difference in side walls construction (plaster vs concrete) which is obviously audible. Hope it would help with that.