You said:
My speakers are modified by the way by heavy concrete load yes but also by some Helmholtz devices attached to them and on top of them (psycho-acoustic method here)...
Concrete as a damping material?
Let’s assume your speakers are already well damped. A good design by the manufacturer. You say I’m simplistic, but you have made a complex mess.
You also have 2 floor fans in your room alongside kitchen pans. Does this also improve the sound? And what does the aluminum foil in the corner of the ceiling do? That all could have been used in a nice toaster oven to warm up pizza...or create a wifi range extender.

