Great suggestion, Frank. I would just add to this that if one needs to venture into any "room treatment" that you can use BOTH absorption AND diffusion to effect improvements. If reflections predominate in the problem department, simply adding some physical contours to any large, flat surface areas can help. If reverberation predominates, then some absorption may need to be added. In the end a combination of both is likely, but often diffusion is my first choice as it doesn't mess so much with the overall sound spectrum. A lot of absorption can kill the top end and make a room dull and lifeless.
Diffusion's also a better solution in rooms used for congregational singing. Singing in a room with too much absorption just starts to feel really weird.
It's not always just too much absorption but often too much absorption at certain frequencies and/or absorption located on the wrong surfaces. I do agree that a combination of diffusion and absorption is often the most effective solution, as well as making sure the materials used are appropriate for use in public assembly spaces with proper flame spread and smoke ratings.
