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

Acoustic treatments were the single biggest improvement and it surprised the heck out of me. I think I had held off because I wasn’t sure how to go about implementing an acoustic treatment strategy and it didn’t seem like a good investment. Boy was I wrong. I’m now of the opinion that acoustically treating your room should be the first step not the last. 

Diffraction and absorption panels, speaker positioning (7 feet from front wall) and, recently, properly dialed in dual subs set up asymmetrically relative to my dedicated room with the help of (gasp) Chat GPT. I am a very happy listener these days.

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