Hello roberto,
Thanks for the answer! I bought them from someone who had them stored for years. They have a nice punch and are quite efficient, the panels are perfectly clean and the mylar is still very tight but one of the speakers seems to lack power in the midrange, it sounds like it plays louder to the back than to the front of the panel and the farther away you move from the speakers, the more apparent the volume difference becomes.
I've already cleaned the connectors and the 3 pads on the PCB as these became heavily oxidized due to HV and age, but it didn't help. They do sound a little bit more fresh and airy now, though.
I also tried swapping the electronics but it didn't help either - the problem is clearly somewhere in the panel, that's the reason I asked if there was another block connector hidden inside the frame, which could've been heavily oxidized too.
Not being sure what kind of CLS I actually have (these are my first ML's) I downloaded the short clip I found on this from on how to remove the CLS stat panels but in the vid it looks like the panel is kept in place with velcro, while mine are enclosed in the wooden frame. I tried pushing them out, but the panel is starting to flex and I'm not sure if this is the right way to do it.
I've been told that it could be the wire/tape on the sides of the panel that is gone bad as the mylar just looks to good to be the problem, it was probably already replaced in the past.