You might also try applying a lower crossover to the right sub and see what that does. It’s possible the sub peak causing the boom is higher in the bass region than you realize. By lowering the crossover it also effectively lowers the volume of the upper bass without touching the lower bass. Can sometimes help.
Also try flipping the sub around so the driver faces the wall or across the wall. The sub itself can be physically large enough that SBIR effects screw up an otherwise good response and the best fix is flipping it around.
I wrote an article for my group here:
https://www.avnirvana.com/threads/sbir-matters-for-subs-too.2269/ that shows an extreme example of it. If you follow along and look at all the measurements you can see what was going on.
One last thing to consider, if the phase of one sub is reversed as compared to the other, it can cancel bass in one region of the response and cause a boost in another. I’ve measured this in other people’s systems and found very boomy one-note bass. Literally a flip of the polarity totally fixed it. I didn’t notice this problem as coming from one side so I don’t suspect this is your problem, but you never know.
The measurement mic recommendation is a good one. I’m brand agnostic so I also suggest looking at the Dayton umm-6.
If budget allows the Cross Spectrum is nice, though I really wouldn’t worry too much about it. The correction to below 20hz isn’t that useful. It’s one of those things that I think serves more for bragging rights than practical usage. Get a mic, whichever one you can justify, download REW, and spend many sleepless nights seeking elusive flatness.
