You are spot on in terms of the cause of the boomy FR - this I am aware from early on;
The on-going challenge is how to effectively address it without introducing issues in other areas.
Theorectically based on the room dimensions;
The primary axial nodes are 43Hz, 55Hz and 67Hz
Comparing to the actual SPL plot the problem area is 43Hz, 67Hz and 80Hz (grey plot).
The 2nd axial node of 55Hz don’t really give any issue, while the tangential node at 79Hz does.
Moving the speakers around I ended up with a relative compromized where the offending 43Hz seems to be addressed. The room treatment adsorbers seems to help 
(as shown by the blue line plot.
After running the YPAO calibration after the latest reshuffling, I’m quite content (for now). The boom is addressed for the most part except for rare occasions while stereo imaging is not compromised too much…
In the future, when itchy finger/butt, things will change again - that’s for sure.