Nathan, the room was designed by an excellent studio architect, which helped. More important than the ceiling, IMO, are the wall soffits which remove the parallel angles, and the 2' thick rigid insulation on the bottom half of two of the walls. The ceilings are angled, so as to avoid standing waves, and in the two spots where it did feel a little bit standing, I did treat the ceiling with some brand of foam tile.
Personally, I think you can get amazing drum sounds with small dead rooms. Terry hit the nail on the head, as usual. Bigger rooms or mics in the hallway just change the perspective.