Can speakers be tested?

I know I'll probably make a lot of people mad by saying this but that’s just the way Magneplars sound. More likely than not, you just got used to listening to other speakers.

UV and humidity are known to cause delamination in older models. I think having strong UV pass though a mylar polymer is a really bad idea. Since they haven't been used in a very long time, its worth letting them play for 20 hours or so before you make a judgement, but I think it is not unlikely that degredation of the mylar and or glue that holds the wires to the mylar has been damaged.

Of course.Take them to a local speaker tech and he will pass a number of sonic tones at various frequencies to see if they are performing up to spec.This will take just a few minutes and you will have your answer.