Finally finished treated the front wall and immediate side walls of my theater room. The entire front wall is treated with 2 inch thick panels of Owens Corning 703 behind the screen, 4 inch on the bottom of the screen, and 4 inch with a 2 inch air gap on the sides of the screen, directly behind the speakers. Hardest/most annoying part of this whole thing was wrapping everything in black fabric. Doing this did help my SBIR problem a good deal, but didn't completely eliminate it. I'd call it a success since it did work, and an added bonus is all of that black around the screen makes for a more immersive image.
Here are some pics of the completed room. While the room is complete, this pics were taken before I had a chance to clean up the room. Disregard the clutter and some of the rough edges on the black fabric. Finishing touches are still in progress.
After playing around with ARC and the quick measure feature, I ended up moving one of my subs to the side wall. When both were on the front wall, the frequency response dropped off sharply right before 80 hz and was left a noticeable gap right at the crossover frequency. Putting one of the subs against the side wall smoothed things out nicely, with a nearly flat FR up to nearly 140 hz.
Check out this video I took of measurements from my left front speaker and how drastically the FR changes as you move the mic about 12 inches to the right of the MLP. This room drives me crazy. What do you think could be causing this? I'm thinking it has something to do with that huge open cavity under the stairs where I have my equipment rack, which you can see in the background of the video.
I took measurements about 6 inches from the speakers, right between the 15" woofer and the compression driver (about level with the JBL emblem). The graph was ruler flat. No dips or peaks at all. As I moved the mic back, the worse and worse the dips and peaks got.