It can come down to several things, my first guess is its quite possible the enclosure is incorrect for them, and causing them to reach they're mechanical limits early. This problem can cause early distortion, bottoming out, and decreased power handling. Your tuning on your amp, What is your box tuned at? you could be playing frequencies beyond the enclosures tuning, causing the same issues i listed above. To blow a speaker, you have to be abusing them, if everything is installed correctly and the gains are set properly, these things wont happen unless the equipment is defective. But you ssaid this is your second set, so there is an issue somewhere in what I listed above.
