| You're over thinking this a little. Basically your room just looks like it needs more broad band treatment and bass trapping. I wouldn't add any more 2" panels, additional air space or not. I'd add as many 4" broad band panels and 6" bass traps as you can manage, leaving an appropriate amount of air space behind each (4"-6"). That will smooth the midrange and deal with the modal stuff around 100Hz.
Frank
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Frank
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