Much good advice given above. Ethan, Nyal and Alex ... all good
Dazzdax, nice image/room, ... thanks for sharing. Those guys are the pros, for sure. But I would like to offer a few thoughts.
I'd strongly recommend
additional placement experimentation of all the parties involved; the Sound Labs, the subs, and perhaps the listening position. I mean just a quick look at your picture, and the floorplan from above, and I see a great deal of possibilities.
You've got a beautiful room, a truly wonderful system, and both appear ideal for placement fine tuning. I'm guessing you've performed some anyway, however I'm confident essentially most all the negative attributes you're experiencing/measuring, could easily be lessened by placement optimization. Fortunately, you've got a room that's conducive to movement away from the front wall. I wouldn't be afraid to bring them out significantly into the space .. allowing more room behind them. Aesthetically, I understand you can't go too crazy,.. but I'm sure there's a combo of front placement, and seating position that would yield superior results.
Ideally, it would be great to experiment with some scattering/diffusion behind the mains. Yeah some proper diffusion products would be ideal, but there are items that would add some scattering, eliciting a better diffuse reflected energy off the back of the mains. If I recall correctly, Sound Lab enthusiast Duke LeJuene, utilizes and recommends something like a Ficus tree (real or otherwise) behind the panels to help in this regard. They're essentially invisible to the LF, yet would offer some nice scattering to the critical octaves above.
The subs; I'd use two of them midwall, on each side and somewhat rearward of the listening position. The other two, equidistant from the rear subs, yet in front of the LP. With careful relative adjustment of subs-to-mains, the sonic "wholeness" or the fabric of the LF will be perfectly interwoven into the mains output (which obviously is the key integrating stats and subs). This will require careful time/signal alignment, careful yes but it shouldn't be hard to tell when it's right. Some iteration of the four separate subs, placed accordingly, will yield an ideal final FR at the seating.
As you most likely know, with such a multi-sub set-up, you'll benefit in both the time and freq domain. The key to "bass speed"
, I've found to be entirely rooted in two things; time alignment of the LF to the remaining spectrum, the LF decay characteristics of the space. You could really use a great deal of bass trapping in your gorgeous room. But, oftentimes bass damping and the high performance gains that come with bass traps is contra-indicated to aesthetic concerns. Maybe you already have some bass trapping. Remember, the subs "see" the entire volume of room air, and although some areas would offer superior performance to others, they could be carefully disguised and placed in a variety of locations. Nulls, like the one you're seeing, come from destructive interference. Bass trapping both lessen this interference, and tighten up bass decay times ... which subjectively is quite appealing.
A rooms standing waves/modal behavior is primarily established by it's dimensions. However, how significantly the standing waves are excited is a function of how the LF sources are located. With four spaced subs, along the long dimension yet well out of the corners, they're very likely going to exhibit less modal influence and more direct sound. There will be one LF source per room quadrant (as seen at the LP), and the FR will possess further smoothing merely by being closer to the LP (just like a close mic measurement, more sub, less room).
Another consideration with the current placement approach you're using, the rear wall behind the listener is enormously significant. Like Ethan mentioned, the rear wall is important in nearly any set-up. However, by spreading the subs fore and aft, left and right, and by bring the mains further off the front boundary, the rear wall's significance is diminished.
Anyway, it would be a pleasure experimenting with those stat panels, those subs, .. in that nice room. Maybe after all the experimenting/measuring, you'll discover another set of placement ideas. But what I layed out is the approach I'd pursue given the latitude to do so. Maybe I'm way off ...
Best of luck