I'd bet, as well, that if you switch your right lower speaker driver it will blow again. I'm pretty sure its not your amp, though. I had the exact same speakers. I bought them new, when they were current. My best friend bought the same exact speaker because he liked mine. My friend that works in the audio store had the same exact speaker. The store I bought them from had 2 brand new pairs on display. (One pair in a 2 channel setup and the other and the other was in a theater.) Every single pair of speakers I just mentioned had the right lower driver go bad. To this day no one can figure out why they went bad.
I was able to come up with the most plausible theory as to what was going on. Understand, though, I may very well be wrong and that this is just my best guess. The only thing common to all 5 pairs of speakers, besides the fact that they were all the same exact model, was the demo music. We all used many of the same CD's. Either we each owned our own copy or we lent each other some albums. I figure that there was something on one of the CD's, like a certain note or sound comming from right channel, that the speaker just did not like. Maybe a crossover point or a weakness in some of the material. Your guess is as good as mine. That's the best I can come up with.
I know that you are going to ask me what CD's we were using. We never figured out which one it was (If it even was one of the CD's to begin with.) There were well over 50 CD's that we had all used. Also, it was a long time ago and I know I can't remember them all.
Sorry that I couldn't give you more info but maybe something I brought up can lead you in a direction that my friends and I overlooked.