This might sound ridiculous, but as you know, the M60s have two sets of terminals( M22s and M50s have one set)to allow for possible bi-amping or bi-wiring. Is it possible that somehow you disconnected the two sets of terminals and that the tweeters aren't connected? Other than the unlikely coincidence of both tweeters being defective, I can't think of any reason for what you report.

-----------------------------------

Enjoy the music, not the equipment.