There is a fair chance that someplace along the line you have acquired a high frequency oscillation that is feeding enough power to blow the tweeters. Sounds like it may be beyond your limit of hearing. It would be very interesting to hook an oscilloscope or frequency analyzer to your speaker leads and see what is there.
The source could be your cable. You say this is Monster cable, but is it shielded? A 25 foot run at preamp line output levels could be picking up RFI and feeding it back to the amp section that powers the speakers with tweeters.
There could also be another reason for this oscillation. Whatever it is, though, you need to find out before you lose another set of tweeters. This is best accomplished with some test equipment, so see if your dealer can help you.
