Interesting. My room is 15.5x23.5x9.5, about 3500 ft^3. I always thought it was a fairly big room for a house. Size makes it easier to adjust, I think, but I guess I've never tried a really big room.
I'm using two ULS-15s & HB-1s in the front corners. The room has some interesting standing waves but I was able to get it down to a 5db range using Room EQ Wizard. It would be 4db but there is a peak just above 20hz I can't seem to control. I don't think I could do this in a much smaller room, but maybe I'm wrong.