Ceiling. Huge. Think of symphony hall. When designing, the ceiling isn’t a constraint like most people’s listening rooms. The changes need to be structural like a soffits in the right places. Or at very least large and otherwise effective hanging mass that brings the room together. Like a great rug. And yes. Rug on floor. Wool. And heavy. Think now about what the musicians have in their studios and up on stage. So many cool rugs. And maybe a pillow or two in the drum set. For walls and such I prefer heavy drapery augmented by music related art and such. Album covers are not ideal. The flat surface even at 12x12 can be a problem. Placement of such memorabilia is critical.
Treating the ceiling and floor, who else has had great results?
Two areas of the room often neglected by audiophiles IMHO is the ceiling and floor. We focus so much on first reflections we forget about overall energy left in a room after the speaker has stopped.
I've had excellent luck with treating the ceiling, especially for home theater applications, and this was before Atmos. The area behind the speakers near the floor often hides noise and distortion which we didn't know we were hearing. Throw a blanket over there and listen for yourself.
Who else has gone through the trouble of treating their ceiling?