Again, I think if the speaker is "farting" and bottoming out, that was not caused by a poor speaker...something MADE that happen to the speaker... Better speakers can typically handle more, but that is not always the case.
Something like a bad signal from the amplifier. Did you have the mids/highs on an external amp. or just the radio power? Distortion blows speakers in a heartbeat.
The "farting: sound could have been the speakers trying to reproduce sounds they are not necessarily intended to do...LOW LOW bass is not going to come through a normal 6.5 speaker with the proper authority like it would a 10" woofer....designed for different things... Possibly the crossover point was too low...?

Not too sure, but Im only trying to throw out ideas/thoughs. Additioanlly, maybe some of this info will prevetn this into the future.

John