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

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.

Because I do not (yet) have dedicated room...

Power cables

Speaker/amp matching 

Speaker/ear height 

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.