I know I'll probably make a lot of people mad by saying this but thats just the way Magneplars sound. More likely than not, you just got used to listening to other speakers.
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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.
Sunlight(UV) does not degrade Mylar, BUT will deteriorate the glue, holding the wires to the older models. Magnepan mentioned that fact in their User Manuals, and cautioned against placing them in sunlight. If that(glue failure) is the case with your speakers; you'll hear them noticeably rattling during play. If not(as mentioned by the first respondent); yout ears have probably just acclimated to the sound of your other speakers.
I've repaired Maggies in the past, by carefully cleaning the damaged(deteriorated glue) areas with acetone and Q-tips. Then very slightly dampening the wire/mylar juncture and applying Gorilla Glue, with a toothpick. The stuff expands like crazy, as it cures, so- I wiped everything but what was actually under the wire away(immediately), with a clean, damp piece of diaper(no lint). Some sections of wire would lift away from the Mylar. In those areas; I used a small piece of Scotch tape(with the end folded over, for easy removal) to hold the wire down, applied glue a bit short of the tape(both sides of it), and came back to it(removed the tape and finished the stretch) after the glue cured. The stuff holds like a gorilla, is extremely light when cured and remains pretty flexible. You have to look at working on speakers(in general) as a labor of love. It takes an extra dose of patience, when Maggies come unglued.