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

I started out 6 yrs ago.  The Gon was not as helpful and I didnt even know the way to ask the question.. step by step I added this, changed that.  But I  would say the single immediate improvement was upgrading the streamer andbusing USB..  But all the other changes and improvement were worthwhile.

XA25, Room Treatments, isolation, new rack, Electrical filters. dedicated room.  Very good punch list!

For me it was adding a distributed bass array (swarm). Solved most of the major problems with the low frequency response quality of my room. Now all I need to do is "tweak".

I have been lucky and always owned since the 1970s reasonably priced quality turntables, arms, cartridges, amps and speakers. Each change incrementally improved, but the TWO most standout moments l most appreciated came more recently.

1, Buying my first sub, a Rel Quake over twenty years ago. It was entry level but what a resounding success story. There were no doubts with instantaneous approval.
2, Upgrading about six years ago from twin Quakes to a pair of Rel T9i’s. Again a massive leap forward, and presently leaving no feelings of a need to improve on them.

Room treatment (DIY), placement of speakers. --- Pearlacoutics Sibelius speakers, getting REL T9 dialed in right. --- Zesto SUT w/ Rega Naia

Added Kimber kable 8tc speaker wire  Made a huge difference.