Sounds like the fuse.
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Listen closely(physically) to the high freq section of both panels, and determine if there is any output. It could very well be that hearing has changed, or that you have become accustomed to the repsonse of your newer speakers. It's unlikely that either the fuses, or hi-pass filter caps, went bad in both speakers.
I had a similar experience when I first got back into audio after 15 years or so. It turned out it was mostly me (my ears), partly my system needing to "break back in" after not being used for so many years and the addition of a power conditioner help a lot. After some research I found out that the power here in the Hudson Valley is pretty bad. My system used to sound it's best in the late hours of the day and sometimes unlistenable during the daytime hours prior to the power conditioner. Try just listening for a while and see what happens.
Yes,its been known that electronic's setting around unused may have electronic parts beside mechanical parts that will settle into different values and change sonically.Dirty power is getting fairly common.Mine is a nightmare.Talk about ears changing,what about those people that have cars stereos you could hear blocks away.They might be wearing hearing aids by the time they reach their 30's.
While the cover are off; Make certain the fuses are good, take an ohmmeter, and touch either probe to the negative speaker terminal, and the other probe to the capacitor lead on the HF panel side. If you have a reading there(around 6 Ohms and perhaps an audible response); the HF panel is good. If the panels and fuses are good; it could well be the electrolytics went bad somehow. Where are you located?
The change that the capacitors are gone is almost zero. Do you have some coloratians? Try to wipe the tweetersection with your fingers. I am sure that you find wire(-s) has/have broken. Need manufacturer support or buy a tweeterkit from Magnepan. (Replacing the original caps with polypropylene does miracles! )