What happened to my room acoustics

In my opinion:

 "There is a roll-off before 20kHz.."  This is the shortest wavelengths being absorbed by surfaces in the room. 

"A dip is present around 12kHz.."  There will be a on-axis cancellation dip in the response with a round horn or round waveguide, centered on the frequency where the mouth reflection arrives 180 degrees out-of-phase a the listening position.   The center frequency of this dip changes with listening distance or microphone distance.  Off-axis the arrival time of the horn mouth reflection is smeared, and the cancellation dippage is correspondingly reduced. 

"There is a noticeable boost between 50Hz and 1.5kHz."  The midwoofer's pattern is wider than the waveguide's pattern below the crossover frequency resulting in more in-room energy below the crossover frequency.

Duke

Thank you all for the advice. It appears that the consensus here is that I am sitting too far from the speakers. I've adjusted the seating position, bringing it closer to the speaker plane (11 ft), with the speakers now placed 9 ft apart. My room has a 10 ft high ceiling. At the new MLP, I measured the spectrogram again on both channels, both together and separately. The FFT results are as follows: The high-frequency dip has shifted to a slightly lower frequency and appears to be more pronounced. The left and right channels exhibit similar profiles.  On paper, it appears that sitting closer is not resulting in improved sonic performance in my room.  Basically, I agree with @audiokinesis who commented on the in-room energy below crossover frequency (=1.8khz).  I probably need a lot of panels on the sides, which may not receive WAF certification.

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.

Thank you, @newbee.  I did experiment with varying toe-in angles, ranging from straight to pointing towards my shoulder, to find a trade-off between the width of the soundstage and imaging. I ultimately settled on a slight toe-in that points slightly outside my shoulder without realizing that what I perceived as an 'augmented' soundstage was, in fact, the out-of-phase reflective wave. Surprisingly, I found that I actually prefer a wider and more three-dimensional soundstage, even though it doesn't come with the benefit of deepening the soundstage that I've always wanted. Dr. Toole has asserted that these reflections, including the first lateral reflection, should not necessarily be absorbed.

I did measure the right channel at 3 ft but didn't post it. The response appears flatter, but it has more humps and dips. I'm beginning to doubt the accuracy of the measurement tool I was using...

A suggestion, I start with math for positioning my woofers (Cardas formula) which in your case will be approximately 6.25' from front wall and 3.8' from side walls.

I would then start sitting (on a chair on casters) at a perfect equilateral triangle and roll back (away) from the speakers slowly which will help you find several "semi sweet" spots which you can focus on later on when fine tuning.