Horning Eufrodites - help needed with boomy bass

I've been reading this with much interest, and kind of surprised by the fact, that the solutions being offered, only address the speaker, as a source of the boominess.
From the OP description though, it seems pretty obvious, that there is a significant room- related issue.

"Very clearly the issue comes from the left side of the room. I'm not sure why. There's a glass door there leading to a corridor which some experts told me would be where the problems comes from but I've covered the door with 3 types of fabric and carpet and it changes absolutely nothing, nada, gurnisht, niente.
There's also a little wall of about 1 meter that comes into the room on the left side about 3 meters from the back wall which creates some kind of a so called "niche" there on the left and when I stand there the boominess is heavy.
When I stand next to the right speakers, there's no boominess at all"

Before you do anything with the speakers, I would try to address that bass buildup in the left part of your room.
Put some crude DIY bass traps in that space (roll of fiberglass insulation?) just to test the theory. If that alleviates the issue, then you are on the right path.
And after the room- related problem is addressed, only then you can play with the speaker port to fix some (if any) residual issues.

Speaking of grounding speakers- I just wanted to share my personal experience, that doesn't necessarily will apply to any other speaker/system.
I have tried multiple approaches- spikes, cones (Mapleshade, Audiopoints, BDR, etc.), isolation platforms (Mapleshade maple blocks, Sistrum, Symposium, Herbie's), etc., etc.
Now my search is (hopefully) over- the best solution by far is Stillpoints Ultra 5s on Symposium Super Plus platform.
The results are simply amazing- increase in resolution, articulation. Very natural and relaxed at the same time presentaion, without any downside, as I experienced with other support systems.
The whole listening experience is at completely different level- I get involved in every single listening session, regardless of what is playing- something, that rarely happened before. And that's what counts.
Again, your experience might not replicate mine.
Should I mention how much I hated to pay the admission price? Absolutely ridiculous if you ask me! But the funny thing is- it was totally worth it.
YMMV

Like I said, bass traps

Tom is right,
I have done much more extensive structural changes in my room in order to tame room bass modes. It is a difficult decision, but by looking at it, shouldn't be that difficult to implement technically- and, you will thank yourself later.
Without some serious re-building you will be "doomed", as far, as the bass goes