Hi thanks for your suggestions. How would you prefer to notch out the high Q resonance(s)? I've been thinking about getting a DSP but it is adding another digital device, sometimes with proprietary software, so I think something DIY op-amp based sounds better to me. (although a DSP gives more flexibility and options) I think correcting the source signal rather than the loudspeaker is a preferable idea as damping screens always affect high frequencies.
There's some good news too: yesterday I finished my second new loudspeaker and I didn't expect an improvement in sound quality but to me it seems these smeared mid / high frequencies and the un-tight lower frequencies have disappeared. The dull, disappointing sound isn't there anymore. I was a bit disappointed as it took many hours of labor, although it was like meditation to me, those repetitive tasks. It got me away from my computer screen which is a good thing Maybe just 1 loudspeaker was exiting room modi, or the small and big stat were out of phase?
The plans are made by myself although I read many books and articles about esl theory and construction. The idea is to divide the stator in multiple segments to tune frequency response and imporove high frequency dispersion and maybe amplifier load gets a little easier too. The dimensions were determined by outer dimensions of a door. The stators contain over 1 km of wire.
Here are some photos I took during construction: