Given your significant limitations and the severity of the issue I’d suggest looking into some form of room correction from the likes of DSpeaker, MiniDSP, etc. Best of luck in taming the beast!
My room is about the same dimensions and my last sub. , a REL was used with a Velodyne SMS 1 . It pulled down that nasty peak. The sub that replaced it has built in DSP and it also does a good job of getting rid of the boom
Before you do anything, consider an Auralex SubDude isolation platform. Cheap and effective at taming resonance. Start there then explore DSP solutions.
Last speaker was the JBL L82 and even the KEF LS50 Meta (which I did not think even went to 40Hz) does it. If I plug the speaker ports, the resonance goes away but so does too much of the bass.
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