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

For me it was new speakers, speaker placement and positioning and room treatment. I have 3 more bass traps on the way now so I'll make additional measurements and listening to see the impact. When I separated the distance between my speaker by another 2-3 feet and moved them another foot away from the back wall, that made the biggest difference.

Two changes spring to mind.

The first was upgrading from an Esoteric X-03 SE to the K-01XS. While the tonality was not changed there was a huge difference in resolution - a layer of absorption just removed.

The upgrade from Koetsu Black to RSP was significant, but the phono stage upgrade from Sutherland Duo to Dos Locos was (hate the phrase) jaw dropping.  Every aspect of the sound was improved immeasurably - more of everything.  If you have a suitable cartridge this phono stage must be auditioned.

At a high level: bass control

Biggest factor in bass control: big Gik soffet traps in corners behind speakers

First would be reclaiming the kids "wreck" room once they all moved out and turning it into my office/listening room. Then letting my wife line each of the long side walls with floor to ceiling book cases filled with books (1000's of books). T-bar ceiling and wall to wall carpet. Acousticly silent. 

Second would then be DIY single driver speakers... OMG the soundstage is now amazing. So much so that I now know what people mean by "sound stage".

Finding this forum. Reading and learning from each of you. 

You will never know how many of these posts I find something I have never heard of before and copy it from your post and search for it on the web.  
 

Of course, this means spending more money….