As I've suspected for quite a long time, our room gain profiles are quite similar. And, now that you're posting far more accurate measurements than many from the past, I can say it without being tarred and feathered.
The low end bump in your close mic is the measurement systems noise floor because the level is too low below 10 Hz. If you bump the level to 110dB at the top end, the roll off should flatten out. I also wonder if you're using the default Tukey window and whether or not you're using a SC calibration file, both of which will tend to skew the result down low.
As I've posted in the past, it's my belief that a loopback measurement of the SC does not necessarily (and in fact most times doesn't) reflect the SCs effect on the measurement because you take the analog input and loop it to the analog output. Since all electronics have DC blocking caps and they are almost always in the analog output stage, the loopback reflects that. But, when you take a measurement, you don't use the analog output. The ADC sends it in the digital realm to REW and the analog output is out of the loop. So, a SC correction file can add bump where it doesn't belong.
Also, back when, M Bentz razzed me about my ACO Pacific hardware being inaccurate below 30 Hz because he misinterpreted the dBA specs. But, during that discussion between the 2 of us, he brought up windowing. I studied the subject fairly intently as a result and have experimented exhaustively since then and found his point about Tukey below 20 Hz to be a valid one.
In any case, the graphs you posted are accurate enough to compare native vs at the LP in both our rooms, which I've done below. All green traces are yours and all gold traces are mine, overlaid on each other's graphs:
I believe that every room has a more similar than not room gain profile and that the 'pressure pot that begins at the longest dimension that can support a standing wave' theory is bunk.
You gain 2 advantages over me: 1) that your masonry walls and floor exhibit far less transmission loss and 2) you have no stairwell or windows and that gives you higher output with less displacement. I gain 1 advantage over you: my listening space in on a huge 27' x 27' wood structure floor system, supported in the center by a Lam-beam, that provides a bump at around 6 Hz. It's a pretty good trade and brings us fairly even, IMO and according to the posted evidence.
I wish you would download my SpecLab settings and get some SL graphs fired up. It would be interesting stuff.