In my experience, acoustic treatments made a rather significant improvement in my small (12x16 and 13x14) listening rooms. Stacked GIK 244s (w/diffusion plates) in the front corners, two of the same ones on the ceiling, 244 w/o plates on 1st and 2nd reflection points and centered on the front wall. And finally one behind the listening seat.
I didn’t put them in one by one and listen to the differences. Everything went in at the same time, except the ceiling, and before vs after was quite dramatic in my room.