I strongly suspect the problem may be with the amplifier and not with the speaker. The Thiel 3.6s are terrific speakers, but because of their occasionally very low impedance are quite difficult to drive. The manufacturer recommends Classe, among other designs that have sufficient current to handle 2 to 3 ohm loads. I would imagine a Bryston 7B-ST would also be satisfactory, as might a Sunfire. But if you attempt to play the speakers at loud levels with a less than satisfactory amplifier for this specific application, the amplifier is apt to clip (which is generally the most prominent cause of tweeter and/or midrange failure) or perhaps even go into oscillation, which would also burn out higher frequency drivers.
