can a sub be blown due to underpowering?

Well, no, chi-pimp, that may be a bad example.  If a sub is capable of getting 1000 watts all day long, then an amp that outputs 300 watts will never give it enough heat to ruin it, no matter how severely it's clipping.  What follows is from http://www.bcae1.com/2ltlpwr.htm:

"Note:

If your speakers are capable of handling significantly more than your amplifier can produce, driving them with a clipped signal will not likely hurt them.

If the speakers can handle 3 or 4 times the power that your amplifier can produce, there's virtually no way to damage your speakers (no matter how clipped the signal is).

If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a clipped signal for extended periods of time may cause speaker damage and/or premature failure.

If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a square wave signal for extended periods of time will likely cause speaker damage."

But like Alpine and chi-pimp are saying, it's the act of pushing an amp beyond its limits to get more power to the speaker that causes failure...damage to the speaker caused by heat.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Like someone has put it best, electronics operate on smoke..........once you let the smoke out, they don't seem to want to work anymore.......hmmmm, puzzling isn't it!!!

that ones going on my list of favorite quotes....nice