under power bad??

Underpowering a speaker doesnt harm it at all. The reason underpowering is associated with blown speakers is that when you use too small an amp for a certain speaker, it wont perform at its optimum and you wont get as much output out of it. So to compensate you crank up the volume or worse, the gain. This increases the chance that youll push the amp into clipping, which sends out a whopper of a powerful signal at 2-3 times the rated power of the amp and limited only by its input voltage. Its this overpowering that kills the speaker.

Still, if youre only using 100 watts into a 400 speaker, even at full distorted clip I doubt it could generate enough power to kill that sub. Now if you were using say a 250 watt amp, then it could potentially fry the 2154 with a clipped signal.

Either way, its best to properly power a speaker and if youre not, then its best to play safe and keep the gains and volume kinda on the low side.

You never want to put the bass boost up all the way. For every 3db of boost in a particular frequency, you are asking you amp to double its power in that frequency...... not many amps on the market have that kind of head room when being played at higher listening levels. Adjusting your gains properly is the best thing to do.