Horning Eufrodites - help needed with boomy bass

As described this is the approach to take. Like I said
previous until this is done any amount of retuning will be
masked by the fact that the speaker is Not grounded. Same
thing happens with a acoustic instrument that touches the
floor. In a cello that has increased grounding potential
there is a great reduction of or even the elimination of a
wolf tone. Increase the grounding potential and the
instrument grows in stature and texture before your eyes. The
instrument becomes even more resonant and "alive". I came
here to present ideas and not a product. I prefer for people
to find out on their own. Now that the name was dropped I
have to say I work with Star Sound. Tom

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.

Cones with sharp points are intended to couple the speaker/stand to the floor so that energy is transferred to the floor. If that floor is sitting on something like a concrete foundation, this huge mass will dissipate the energy without that much being re-radiated as sound. If the flooring is wood suspended on floor beams, that might result in the floor itself acting as a kind of sounding board. This could exacerbate a problem with booming. Whether one approach is better or the other (absorption in a compliant pad, or the fooring doing the absorption) is something that has to be tried.

But, before even doing this kind of experiment, you should experiment with room placement. You can sort of do this by random movement, or use a more systematic approach. I sort of like the "Sumiko" approach which emphasizes proper bass balance. You can find directions on the internet. When using this approach it is amazing how a movement as small as one inch will dramatically change bass response.

The next thing I would try are bass traps in the corners of the room. The primary downside to bass traps is that they are often ugly and take up a lot of room. The lower the frequency that needs to be trapped, the bigger the trap that is needed. I use double stacks of 16" ASC tube traps myself and they do work well.

I have not tried equalization in my own system, but, I have heard it effectively used in some systems.

Good luck.

Everything mentioned are steps.The first step should be grounding the speaker. Until the grounding is done there will be no stable launch pad. As for the room itself geometric angles and shapes strategically placed will redirect low frequency and help with standing wave resonance. Tom

Maril555,
Bass traps were suggested early in this thead(there` no harm in trying them) to address room acoustics. There`s no reason why that and mechanical grounding can`t be done simultaneously,this is fundamental physics and the promblem can be solved.
Regards,