Acoustic treatments, first, the first reflection point did so much for mids and treble and imaging, but then years later I moved some panels and soffit traps to block perpendicular walls to attenuate boundary interference, and THAT was what ultimately solved my 25 year problem with bass in a 23.5 x 20 ft room. It wasn't modes at all, it was the cancellation of multiple freq in the bass region from BI. Second best effect was moving 2 subs away from the mains and out into the room but at the sides. Third was a combined ground system, Synergistic Research plus Puritan Ground Master. The combination created a balance that neither alone could.
