How Did It Blow?

Well i dunno.  Im still not really convinced.  Over excursion will over heat the coil because when you push the coil hard enough to pop out of the magnet a little you lose the magnetic core, which will weaken the magnetic field around the coil which will cause it to draw more current to do the work.  Sub sonic filters cut off at   i forget acctually   around 20 or 30 hz i think.  These frequencies arent too awful common unless your pounding a lot of rap with those freq. or one of those BASS cd's.  I never really had a sub sonic problem and i have always run ported baxes with no filter, but i dopnt listen to much if any sub sonic music so i duno. 

As far as the gain goes, you said you had it set low.  yes you should properly set your gains, but with a sub that exceeds the amp specs like that and with a conservative approach, if anything your just cheating yourself from getting the most out of your amp.  Clipping could cause the coil to over heat beause your "over-driving" and amp in its saturation region which means its putting out its maximum power for a large part of the wave form.  Thats not a good thing, but if the stats you put up for the sub and amp are correct, the subs rms power handling should be able to handle that amps max output.  Plus again, if you did set your gains conservitavly like you said, even if it was clipping a little, it shouldnt have been that bad.  If its def. wired correctly (proper channels bridged, no shorts, ect)   i would go b&*ch at sumone and hopefully if it wasnt your fault get you equip replaced.  Good Luck

Here are another couple of things to consider as well. The source material you are playing may indeed have a severely clipped signal in the recording. As you turn up the volume this clipped signal is then passed onto the amp and then the speaker. Next is how you are using the cd player, if you have the loudness turned on, the bass control turned up, then you also may be clipping the outputs of the cd player and no amount of gain setting at the amp will rid a clipped signal here.

Next is a bass boost circuit on a amp, another potential killer in disguise.

Finally and the more than likely culprit. Dude, you have to let a sub cool down. Heat is the #1 killer of any sub or speaker as it is a natural byproduct of producing power. Thus 20 minutes at a sustained volume level may just be enough to melt that coil up real good, especially if it was being fed a clipped signal to begin with.