What happened to my room acoustics

FWIW, my room is about the same size as yours (19.5x13.5x9), but my set up is a tad different. I use the triangulated set up with my speakers 4' off the wall behind them, 9' apart, sharply toed in (axis crossing slightly in front of my head). My chair is about 4.5' off the back wall and placed to avoid a bass node at about 60hz. I have no special room treatments but I do have window coverings (heavy drapes), book cases, and soft furniture. 1st reflection points have been dealt with. My sound stage is excellent in that it allows very specific instrument placement over the entire wall behind the speakers. I measured my room, speakers etc endlessly (it seems) but ultimately it was tuned by ear.

The point of my saying all of that is to suggest that your chair might be too far back from the plane of your speakers which might just be a bit too close to the wall behind them and your low and midrange node may be the result of a room node which will change with moving your speakers forward. The HF dip could be the result of the off axis output of your speakers (but not so much reflections, I think).

BTW, FWIW, I know nothing special about your speakers. Also, for the most part, the measurement that really matter are those made with the mic at the listening position. Measuring your room or trying to duplicate the speakers spec's is really meaningless, interesting perhaps. Regarding sound stage, with my present system, the difference between my present system set up and one with the chair further back toward the rear wall was that the sense of depth diminished and there was some loss in specificity.  Those issues disappeared when I moved my chair forward (towards the speakers) to something closer to an equilateral triangle.

As I look at your latest graph readings, and I do not know how your unit operates, my initial concern is not so much about the rise of the upper bass thru the mid range, as a unevenness of the ’curve’ which shows alternating +/- readings of up to 10 db’s (between 100hz and 200hz and 1khz and 2khz. This suggests to me, and seems to support your conclusions re needing to tame your room with acoustic materials of some sort. But, before you go there, if you do, try running your tests with your speakers axis in different positions. FWIW, one of the benefits (and to others detriments’) of minimal to no toe in is that the 1st reflection points to the sides of the speakers can give a sense of greater width, however in reality in a recording an in-phase sound can only appear between the speakers. What you hear on the outside of your speakers is out of phase sound put there in the recording process or reflections off the side walls. I think you have pretty much identified the differences in set up and realize it’s a choice that affects what you want to hear.

I’m almost out of wind now, but FWIW, you might try measuring the FR of each speaker separately with your mic about 6ft from your speaker and see how flat the speakers FR really is and determine how much its unevenness, or lack thereof is contributing to your in-room response at your listening position. Just for kicks you might try measuring the FR from your listening position with it in an equilateral triangle and your speakers on axis. Probably won’t resolve anything but it might just be interesting. It will/should give you results which reduce the effect of the room in your sound. Hope this helps a bit.