Looking at the Left and Right separately has revealed a problem. Here's the individual MM for the left and right compared to the estimated in room;
There's good compliance above 1200Hz, whilst 280Hz and 850Hz peaks are still prevalent. Also 390Hz is looking like it might need some attention too. But the real problem seems to be 80Hz and 160Hz. For whatever reason, the Left channel is responsible for both peaks. I tried different combos of doors and curtains opened and closed, and also tweaked the position even more accurately (it was already within 1cm), but the difference was negligible.
When I compare MM of both speakers (LR=stereo) vs the L+R arithmetic sum, it's not too different, but actually it now shows a phase issue at ~90Hz.
But when I look at the L and R compared to the LR measurement, those spots stand out.
So what should I do here? The left speaker/wall is basically that of a perfect rectangle room, whereas the right speaker has a window recess as well as the janky "far" corner. I'm doubting that speaker position will help, considering I've accurately located them relative to their corners (<1cm). So should I just put a -7dB PEQ on the left channel at 160Hz? Similar at 80Hz? What can I do about the phase issue at 90Hz? Or have I got it backwards and the left is a better response, and perhaps the right is suffering from its own phase issues?
Actually, I've often thought the left side was a little louder, to the point where I've gone through windows and stuff to make sure I didn't accidentally pan it 1% or something. But it never is. That's left me to think it's either a slightly biased panning in particular songs, or that my left ear is a little more sensitive, but maybe it's just this peak at 160Hz?
Update: Ok, that actually seemed to work. -7dB and 12Q on the left channel pulled it down equal to the right channel. The resulting stereo measurement is now 3dB lower at that point. The 80Hz EQ wasn't as successful (-5dB and 15Q). It actually caused the stereo measurement to plunge in to the 90Hz issue even worse. I wonder if the problem here is actually the right speaker dips at 60Hz and 84Hz, and perhaps they are SBIR issues from the window recess?
And what does it mean when you add the left and right together, but then the stereo measurement is much higher? Is that just confirmation of room mode issues / energy building up at that frequency? For example;
70Hz has L=72.9dB R=72.5dB SUM=75.5dB Actual=77.0dB.
160Hz is L=78.0dB R=77.0dB SUM=80.2dB Actual=82.5dB.