Large-Room Acoustics

Small rooms have modal ringing, large rooms (>10,000ft3) have a reverberant field: that's about all I've gathered about the differences between the two.  While I'm reasonably familiar with how small critical listening spaces need to be treated, how it the approach for large performance rooms different?  I know they're measured with RT60 and that the acceptable times for that purpose range from 1.3s-2.2s+.  Is RT60 averaged over a certain bandwidth like Noise Reduction Coefficients (250Hz-2kHz) or does it usually refer to just a narrow band like 500Hz?  How do you decide between diffusion and absorption and then which kind of each?  I remember Ethan demonstrating the weakness of cylindrical diffusers and yet it seems like people use them in big spaces. 

Maybe it's time I read up on the subject.  This Bob Gold's article on RT60 has some books in the References section at the bottom so I guess I can start there.
www.bobgolds.com/RT60/rt60.htm