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

For my main 2.2 system it was upgrading the speakers.  Arendal 1723 towers and Revel M126Be standmounts.  Next it was upgrading my streamer and DAC.  Innuos MK3 and Holo Audio Cyan 2.  In my surround sound system it was upgrading to the Marantz Cinema 30 AVR. 

86ing cone speakers.

Well with DSP (using Roon) I have pretty much complete control of the sound so I’d say that.  
 

Other than that then use of Ohm Walsh speakers which are completely different. 

Then there is also recent amp and DAC upgrades with huge impact. 
 

Room acoustics is a given, the question then becomes what you do about it.  A proper setup accordingly is always the first place to start. 
 

Here is more info on why the Ohms make such a difference:

The Ohm Walsh dispersion pattern is a 360-degree cylindrical wavefront. Instead of beaming sound in one direction like traditional box speakers, it radiates a coherent, circular sound field that mimics how instruments naturally project sound into a room. [1, 2, 3, 4]

How the Walsh Driver Works

Invented by Lincoln Walsh, the driver is mounted vertically with the cone inverted, apex pointing upward into the enclosure. [1, 2]

• The Transmission Line: Sound travels through the cone material at supersonic speeds.
• Coherent Dispersion: As the sound waves enter the air along the slope of the cone, they fuse into a perfectly phase-aligned, omnidirectional waveform.
• Frequency Distribution: High frequencies radiate from the top of the cone (near the apex/voice coil) while low frequencies radiate from the wider base. [1, 2]

Key Performance Benefits

• The "Sweet Sweep": Because sound naturally disperses in all directions, the stereo "sweet spot" is massive. You can sit virtually anywhere in the room and still hear a cohesive stereo image.
• Room Integration: By radiating 360 degrees, the speakers interact with your room's natural acoustics to create a more realistic, lifelike sense of space and ambiance. [1, 2, 3, 4, 5, 6]

Controlled vs. Pure Omni

While early designs like the legendary Ohm A and Ohm F radiated sound equally in every direction (true omnidirectional), modern Ohm Walsh models—such as the Ohm Walsh 1000 or 2000 Series—feature "controlled wide dispersion." These models suppress radiation on the sides and back to make room placement much easier, allowing you to place them relatively close to walls without muddying the soundstage. [1, 2, 3, 4, 5, 6]

New room construction from scratch. 

This is one of the best threads on this forum recently. No one is showing off and everything is constructive. New-comers to this hobby should bookmark it immediately.

Keep those fantastic experiences coming!