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

Because the room is 50% of the sound: bass traps and first reflection diffusion.

Room treatments made a significant difference but over the years moving up to progressively better speakers have made the most difference. Putting Sonus Faber in my system was a big improvement. I went up the SF line to reference level and it was a significant improvement, to the point I thought I was done. About a year ago, I went to Zellaton speakers and the improvement was astonishing. Retained what I loved about SF but improved speed, bass, and a more natural tone, to my ears. All of which confirms my view that the room and loudspeakers are the most meaningful considerations in system building, but there are many different opinions on that point.

When we purchased our current home, I was thrilled to have a 12'x24' room as my media room and a place for my trusty Magnepan 3.6r speakers. Only problem was that the new room sounded horrible.

What finally changed it for me was purchasing two corner bass traps from ATS Acoustics and placed them on the front wall corners behind the speakers. Best $600-700 I ever spent. There was an immediate and profound difference in what I was hearing. Vocals were clearer and bass was smoother and more even. I was now able to listen for a few hours at a time whereas before, I would get listener fatigue after about 45-60 minutes.

Since the bass traps did wonders, I picked up matching absorption 2'x4' panels for the side walls along with 3D printed diffuser panels I picked up from an Audiogon listing. All of this made an amazing transformation and didn't cost an insane amount of money or require a lot of effort to install. Best part about this is, that it works with any equipment upgrade I may make in the future. Wish I had done this sooner.

Adding to my post above, I have to give an honorable mention to Nobsound springs. I have them under almost everything. Little money, big improvement in holography.

In descending order:

Acoustic treatments

Electric quality

Decoupling products