Excellent. That's all you can do is take it one step at a time. Position is key and king.Thanks for the advice - I plan on doing this in several stages. I'll measure the empty room to find the best listening position (the place where I need to fight the low modes the least), then treat the front and back walls first. I'll listen again, measure, then do the ceiling. If I start to dislike the room, I'll back off.
Diffusion does require space to be effective. In most situations it's at least 4' to 6'. Though it varies depending on the lowest frequency you are trying to diffuse. The room is big enough that you should be able to take advantage of diffusion along the largest dimension.I saw in one of the studio construction videos, that diffusion was strategically placed in locations that would scatter sound from the listening positioning, but not from the speakers. I could absorb all the first reflections, but leave diffusers everywhere else. I could even extend the diffusion to the planes above/below ear height (then maybe vertical scattering with a 2D BAD might be more useful than horizontal alone).
A dead room happens when there is too much absorption. If the absorption isn't linear or consistent across the entire spectrum it will cause the room to sound imbalanced. The easiest way is to have too many thin panels, but it can happen with larger ones too if you say covered every possible surface.My understanding so far is that a dead room happens the there's uneven absorption - i.e too many thin absorbers that kill the high end, but barely touch the low end. This, unless I've really misunderstood things badly, would result in increasing amounts of sound reaching you, the lower the sound frequency is, making everything sound dull and muffled - like the tweeters have been smothered with a pillow.
You want the direct sound to reach you and decay at its natural rate. This is what absorption and diffusion do. They dampen/scatter the reflections lessening their impact on the direct sound.
GIK has a ton of options for limiters and more targeted bass traps. I used them in the past and was very please with them. You can contact them through their online forms and a rep can guide you through which products are the best.My hope is that by using deep absorption, everything will be absorbed equally down to the low-ish bass, meaning at worst the room would sound boomy with bass-heavy audio, but not dead. To deal with that my plan was to aggressively tackle the low end with membrane traps and EQ, whilst trying to recover some of the highs with diffusion. Those are things I would add at the very end however. Measure, check where in the room the particular mode has high pressure buildup, buy membrane traps from GIK (I don't think I'm skilled enough to make those), treat the appropriate wall (only rear and side wall, then measure again.
Step by step, that's the way to do it. Document each one. Would be a great adventure for the forums and a good guide for people later on.Once I'm happy with the sound, paint the room, furnish it, and pray EQ/Dirac can deal with whatever is left and whatever damage the furniture causes.