By far the most notable tweak that made a huge difference and it cost nothing.
Getting a better understanding of the bass nulls/dead spots in my room led to far better bass. Simply learning how the sound was affected by the location of the subwoofers in relation to my seat. The best result came when, out of frustration, I did the unthinkable and placed one sub directly in the corner and the other mid-wall sub both turned 90 degrees inches from the wall. "but you're not supposed to do that!" It removed the null in the middle of the room and the bass was improved through out the entire space. Tight and dynamic levels are easily controlled by gain control. The improvement was a major transformation.