A long time ago I noticed a big improvement moving my speakers so the backs of them (which has ports for the bass) are 3 feet from the wall behind them. It was surprising because I didn't move them out all that much. Maybe 6 inches. I have a credenza between them that is about 2-1/2 feet high which probably increased the improvement a lot since the speakers are now an inch or two in front of the credenza.
FYI- the sound also improved a lot when putting my speakers (KEF R500s) on top of multilayered Symposium acoustics stealth platforms. Supposedly it cleans up the vibrations inside the cabinet while also isolating it from vibrations from my suspended floor. All I know is it sounds clearer and more focused.