Ususally when you under power a sub the distortion levels are going to be significantley higher and when speakers play at high distortion levels the speaker life seems to be shortened. In the experiences I have had it seems it is always better to overpower than underpower. You can always turn too much down, but cant turn too little up.
But when you underpower a sub, it has no ill effects.
A sub is nothing more than an AC motor. One of the few ways to kill an AC motor is to overheat it. Since you are putting less watts to it, there is no way for it to surpass it's thermal design limits, right? Even distorted.
A speaker does not know if it is playing music or distortion...they are the same to the motor.
Now, if you are talking about clipping an amp (like when an installer sets the gains to give the customer the capability to clip the amp), and that causes the speaker to recieve watts past it's design limits...then it can overheat the motor.
However, it was too much power that killed it...not enough.
I agree that overpowering subs tends to blow them less. However, that is usually because they are recieving less watts than perhaps a lower output amp driven into clipping, by a loose nut on the gain control.
But I'm an idiot, so consider the source.