What one change to your room, system, or setup made the biggest improvement?

Pulling speakers way out into the room. The entire space between speakers and rear wall became the soundstage. 

The first was adding room treatments, both absorption and diffusion and then it was going from 2 to 4 subs. 

The 1 thing that had the largest impact was using and learning about acoustics measurements as it pertains to ER (early reflections), DT (decay time), and FR (frequency response).  The OmniMic software tool allowed me to (mostly) “see what I’m hearing” which started me down the room treatments path.

I’d say curiosity was equally important as the measurement software as idea after idea flowed and testing followed which resulted for me in a long and large learning curve.  Now I help others analyze and decipher their room measurements and recommend treatments accordingly.

Delta 8 THC gummies.

The listening experience went from wow to omg

Placement and room treatment/acoustics. That should be just about everyone else's answer, as well.