Here's my room, from near the LP with nothing but the sub's internal boost/cut filter. The room is 22' long, so it can't support that resonance below 20Hz... clear indication of a scondary resonance. In my case, it's a 5' doorway in a wall with no corners (furniture), opposite a brick fireplace. The tail in the mid 20's is the lowest normal mode, and the house 2nd harmonic.
Do this in your room and let's see what you get. This is called a "spectrogram" in REW. If you've done a waterfall, you're very close; same data. Key bits are:
- measure loud, so you get 50-60dB above your noise floor
- choose the color axis carefully, so a -40dB decay is easy to see.
- turn on contour lines as well as color
- Include the 10-20Hz octave, so you catch room interactions.
One criteria for a good listening room is about 0.3sec for a 40dB decay. I've scaled the chart so there's 40dB between min/max contour lines. My room meets that criteria quite uniformly down to 50-60Hz, when -40dB decay extends to 0.5sec. I'm not sorried about the infrasonic decay from the rest of the house, even if you are.
The important point to note is that I've added no acoustic treatment
to the room. This is not surprising if you look at data for typical residential rooms.
Since you've treated your room, the first question you should have asked is "what's my room like" right now. The current state of a room is immaterial until a problem is identified. The solution may be to add something, or to take something present away.
I will wager you see room interactions that are likely far worse than mine. Long tails in the bass representing contributions from adjoining space coming from both foyer and dining rooms.
Since you have multiple subwoofers, the first thing I'd suggest is finding the worst resonance frequencies, and then locating the subwoofers to minimize energy injection into that mode. Measure with your largest sub in the left front corner and you should excite everything. Whichever frequencies have the longest resonances are those we want to avoid if possible. (Not everything's possible).
- corners excite all room modes, thus a good place for initial measurement
- mid-wall placement only excites even harmonics along that wall
- 1/3 wall placement excites fundmental and 2nd harmonic evenly, but really drives the 3rd harmonic, OK if that frequency is well above the 80Hz sub crossover.
As to your questions, you can absorb bass energy anywhere it's present. You want to find the highest pressure areas, then use a pressure-based absorption device, not a velocity-based device such as your current panels. These traps like corners because corners concentrate pressure, but they work elsewhere in the room. Same with subwoofers, but both want to be placed for optimum results, not maximum results.
The other thing I'll recommend is measuring in multiple places. This looks like a single-sweet-spot layout, and even the width of your elbows will result in massive response changes. Thankfully we have two ears!
So convert some waterfalls, or post them if you prefer that display. Based on the data, there may be some specific approaches indicated... guided by the result they produce.