Würth 150kHz RF ferrite.
It was trial and error too. I have a lot of in my house, and
Schaffner RF/EMI FN9244B inlet filters too.
If you put the
RF ferrite in the speaker cable (outside the box) -> sound equalized like "
U" -> too much highs

The same effect,
U, with my big 3-way speakers
(woofer: 10.8") of the big room,
kidnapped by TV and family.
I do not have a logical explanation. I asked in the diyaudio.com forums and silence. As some of you already know, I am very inquisitive. Of course, first I try to find the answer for myself before bothering others.
Trust me, with my cheap tweaks and now with more mass these speakers sound very good, amazing. That is why I insist again and again on recommending them before they run out. They are a safe bet. Both for music and as
auxiliary speakers in cinema system (the front bass-reflex is very interesting).