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

in rank order (7 very significant drivers of high level sonic performance)

1 - building a dedicated room

2 - treating said room

big gap

3 - magnepan 3.7x with rel subs

4 - lampizator dac

another big gap

5 - grimm streamer (mostly stream these days)

6 - lan cleansing for streaming

7 - well tempered labs tt / dynavector cart

A tie: Holostages and Telos QMT

When I finally smartened up and switched to McIntosh electronics.  Tube preamp, solid state amp, and tuner.  The synergy is there in spades and I have never felt I need more.  Did it with both my systems.  Yes, it was very expensive but truly worth every penny.   I could have saved many tens of thousands over the years if I bought Mac to begin with but I was always told they were for doctors and lawyers who wanted something impressive and expensive but not that good.  They were wrong. 

After getting the room acoustics, correct, positioning the speakers

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.