Chris, I don't want to clog up the master thread with my ramblings about your theater, but a couple other things have come to mind.
First - on your graphs, especially this one:
You commented that you were concerned about the funkiness above 80 Hz. A) Was this measurement even made with LCR engaged? (I don't mean to suggest that you should be measuring with all the channels driven, but you will be listening that way...) Even if it was...B) can you hear it? Apply 1/3 octave smoothing and see what it looks like. I know that you want it as nice as possible, and maybe there is an argument that you can detect uneven response at finer resolution than 1/3 octave - but concentrate on known confirmed problems, right?
Second - have you gotten familiar with how to interpret the results of room mode calculators, like Bob Golds? http://www.bobgolds.com/Mode/RoomModes.htm
If you input your room dimensions (18, 10.5, 7) there is a predicted strong resonance at 80.7Hz:
"80.7Hz 22.1% 14'0", 7'0", 3'6" (0,0,1 Axial)"
This is the vertical mode of the room. I know that because the wavelength is given as 14 ft, which is twice your room height, and half the wave has to fit any given dimension for it to resonate. This means there is a null in the frequency response at just over 80 Hz halfway between the floor and ceiling. ( I also know that because the numbers (0, 0, 1) correspond to length, width, and height - if the standing wave were reflecting off the front and rear walls, there would be a non-zero number in the first spot. Also, "Axial" indicates the wave travels along one of the three major axes of the room: x, y, or z normally, but p, q, and r for this calculator, at least)
If you give the calculator the room height for areas with risers, you'll see that the frequency goes higher. The overall response in your room will be closely related to the (weighted) average ceiling height. I suspect that your mic was 3 to 3.5 feet off the floor when you made this measurement.
Does that jive with what you know about your room? Measure again with the mic only 1.5 feet from the ceiling or floor - if the 82Hz null goes away, you've found the source.
So, let's assume I've deduced what happened and you get a positive result from measuring nearer the floor or ceiling - then what? What can you do? You need the keep the 82Hz (approximately) wave from reflecting from either the floor or ceiling. Something big and fluffy near where you sit might absorb it. How about a big fluffy chair?
Is there furniture in the room? Now that I say it out loud to myself, I bet this would be a huge help. If that's no good, you'll have to get more creative.
That's probably enough hypothesizing for one post. Let us know if this is ringing any bells (pun intended).