I had a room with perfect sound once. Then I moved.
Took all I learned from that room into the new one. I have good sound now. But not like that room. It was a near perfect listening room.
What made it so great? Without a doubt the acoustic ceiling. It was the early acoustic ceiling made from concrete. No asbestos.
One side was mostly rock wall. So had to fill the other wall with many carvings and sculptures to mimic the other wall.
Anyway, will probably never see a room like that until I can find someone who does concrete spray ceilings. Brilliant. Hope they come back in style.