I agree with Tom on every count except for making Jon's bass traps. They really aren't that difficult.
A well balanced room is no different than a well balanced system. You can't concentrate on one aspect and neglect all of the others hoping that everything else will fall into place. As such, you need to use a little diffusion, a little absorption, etc... as the room and system needs it.
Keep in mind that two rooms of the same exact dimensions might be set up quite differently though. The overall layout and what is needed in terms of absorbers, diffraction, etc... will vary depending on the type of speakers, their placement and their type of radiation pattern.
With that understood, i once again stress that there is NO universal formula that will work in all situations ( just like speaker placement formulas ). Without personalized data on the specific situation, nobody can tell you how a room should be treated. Most of it will boil down to common sense and quite a bit of trial and error. Sean
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