Horning Eufrodites - help needed with boomy bass

"It would be very interesting to compare the "absorbing" product to the "mechanical grounding" method to hear which is more effective in minimizing/eliminating the bass boom."

It's not necessarily either/or; neither excludes the other.

Also, IF lowering the tuning frequency of the port by reducing its cross-sectional area (as described a few posts up) actually works, I think that approach holds out more promise than port-stuffing because it is less likely to have a downside.

Duke

Good morning from Paris.

I'm still not very clear about the what the "mechanical grounding" tweaks are. Can anyone give product examples for those and to the "absorbing" ones?

I'm also not 100% sure I understand the way the wood boards have to be inserted into the bass port. Can you clarify?
Does it matter which wood to use and what the thickness of the board should be?

Re Tommy, I've been in touch with him now for about 8 months and he usually does not provide many suggestions.

Regards.

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.