Your amp is clipping, a very common cause of tweeters being destroyed; I myself have done it ;-(. All Magneplanars require huge amounts of current, the 20.7 the most of all models (but not more than the woofer panels of my Tympani-T-IVa!). Take a look at the Sanders Magtech, an amp made specifically for magnetic-planar loudspeakers.
Maggie 20.7's and Tweeters
I've been a Maggie owner for years. Had a 3.7R for a long time. Then a couple years ago I bought 20.7's I've blown the tweeters twice. One time when having a "battle of the bands" with my neighbor - which I can understand. But just recently this happened on BOTH tweeters and I don't, in the normal course, play my system that loud. I'm a little perplexed. I blew a tweeter out once with my 3.7R in 10 years. It seems the 20.7 Maggies go through tweeters. Any experienced opinions?