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

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.