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

After I bought my Harbeth speakers I made some diy bass traps and put them on the wall behind the speakers. Music just jumped out at you. That's the best description I can think of. I did a before and after with my son one day. He heard it too. It wasn't subtle.

Vibration control on the turntable - putting a Townshend platform on top of a wall shelf to hold the turntable.Double treatment yes, but more than worth it,

Probably was my speaker upgrade, but that was a few months ago. My most recent change: Transparent Speaker Cable Plus > Ultra was significant. Upgraded Moon amp and Ultra interconnect arrive on Monday. Looking forward to the change.  

Activating the "COMP" button on my integrated amp made the music come alive, less veiled.

Moving the speakers, to the right  possession.