Chris, let me preface this by saying that I'm probably newer at this than you, but I've been doing a lot of reading and internet-searching. So I've got a thing or two to say that hopefully will be helpful. ...I hope it will help, because then I can copy what you do for my own room
First, look again at your waterfall graphs. You'll see that the sound persists the longest at a few of your low bass (and very low bass, but not too much infrasonic, IMO) peaks. This contributes significantly to the "boominess" you experience. Remember, your room, like a bell, will ring. The notes the room plays naturally (it's modes) will sustain for a long time. Even when this ringing is quiet, because you've turned down the gain of the signals at those frequencies, the persistence of the sound remains present in your bell of a room - it's still ringing and still muddying the response. My point here is that you can do a lot with EQ, but you can't remove energy once it's in the room - and that is much of the problem in small rooms. Making the bass energy decay at a rate similar to the midrange is a serious task, but it's important. The effective way to do this is with bass traps.
For that reason, Ethan Winer always recommends more bass traps. I'm with him. You can cover them with reflective whatever and EQ back in some higher frequency later if you need, but that bass energy can't be allowed to persist that long, IMO.
Here's one of your graphs you posted a couple weeks ago:
You've noticed, I'm sure, that most of you graphs show the same peak around 63 Hz (my guess of the peak center frequency) which I think you'll find corresponds to the axial mode for the length of your room - somewhere around 17 to 18 feet. In fact, I put your room dimensions into BobGold's Room Modes Calculator
, and the first major peak it predicts is: 62.8 hz 0.7% 17'12", 8'12", 4'6" (2,0,0 Axial) I suppose that makes sense, since your subs are built into the front wall. If you use porous absorption when you tame this problem, then you'll probably have gotten the rest of them too. (There's also a predicted tangential mode at 62.3 Hz, so you've been double-whammied in that small range.)
Now, in terms of specific recommendations for your problem... it seems you don't have the floor space (wall space) to give up for too much porous absorption. Still, it's the best bet, in general. That said, their may be some good alternatives. Ethan Winer's bass trap article
shows how to build some traps that are very small compared to what you would imagine recommended for pure porous traps. He does use a lot of them, however.
He makes a few important points in that article which have significant bearing on your room. First, and most significantly, he issues this warning at the head of the article
Please understand that the bass traps described in this article are intended mainly for larger rooms. For rooms smaller than 25 by 15 feet you'll do better with broadband bass traps made from thick rigid fiberglass placed in the room corners. Small rooms have peaks and nulls at all frequencies, so broadband absorption straddling corners is better than the tuned traps shown in this article. This is described in my Acoustics FAQ linked above. Once all of the corners are treated, then it makes sense to consider adding bass traps like these flat on the walls.
Second, his note at the end off the article (Added later) further limits the application of this smaller style trap:
Although it is not stated in the article, the low-bass trap is effective at frequencies between about 80 Hz. and 160 Hz.
That said, I bet that if you were to cover as much of your rear wall as possible with these (less than four inches thick) traps, you'd be well on your way. Further, figuring out how to tune these a little lower is a much more "researchable" problem.
The other solution, and one I hope to be able to use to some extent, is Helmholtz absorbers. If you read at Gearslutz and elsewhere on the internet, you find mostly failed attempts at tuning them effectively. Also they tend to be very narrow in their effective frequency range, not to mention sensitive to positioning within your room. This might be a last resort.
Hope that helps...