Thank you Roberto!Hola Phil,
Yes, that's a bad sign. When the stat panels start to become flat and muffled, you must, before changing the panels, to do the washing procedure. Many of us, users of ML speakers, have done this with good results. If after the washing procedure are still flat, it is time to think is new panels. Your Aerius is a model at the beginning of the 90s, they are going to get 30 years old. Also, they are a wonderful sounding speakers. The only problem that I see, is the price for a new fresh stat panels. Keep in mind that if you decide to go for a fresh pair of stat panels, they come with a five year factory warranty.
I am sorry to be the one telling you a bad news. Here you can find how to do the stat panels washing procedure: Martin Logan Shower Cleaning PDF