I always shoot for about a 20 - 25% higher power rating on the speakers than what the amp delivers. Yes it is true that if you have a seriously underpowered amp you can screw up the signal coming from the amp and blow a speaker. I have never blown anything up like this, but it can happen . In the old days this was something that never happened..... it was always too much power that got got ya 
I'd like to hear Psycho Bass Guys take on this....he actually knows what he's talking about whereas I'm just a guy who learned a bit by blowing a lot of stuff up .
Rick
Don't feel too bad. I've smoked two sets of drivers in my life; the most recent was a pair of horns in some EV SX300's that had already taken a bad hit from lightning so anything would have killed them, but I spectacularly destroyed a pair of Cerwin Vega SL36 subs when an EQ feeding their power amp had a subsonic oscillation in its output and I had a
real power amp pushing them, a bridged Crown MA3600. They got so hot, the magnets turned bright blue permanently and the voice coils had vaporized; not burned, not charred, vaporized.
For most instrument amps, ratings of thermal max power are pretty trustworthy, however, since we play bass which is:
a. hard to hear relative to most other instruments and
b.hard for speakers to reproduce relative to most instruments
... we get to deal with mechanical limitations of drivers as well as thermal. With porting, it is quite possible to destroy a 1000 watt rated speaker with less than 200 clean watts below its tuning frequency, which is why it's important to pay attention to the repsonse curves of a subwoofer and not try to get 30 Hz out of a speaker cabinet than has a 10 dB downpoint at 50.
Now that I've added that little wrinkle, I've got some good news for the OP; your cab is sealed. That means that any added lows below its resonant frequency will simply result in less volume below that point, and unless you exceed the speaker's thermal limit (the wattage rating) with continuous power, you need not fear hurting the speakers. As far as the old saw of "underpowering," it was only true in the first decade or so of solid state power when poorly regulated amps would often dump all kinds of nasty stuff into their audio output when overdriven and many a speaker got toasted by having an amp with too little power ( and unrealistic expectations of volume).
As far as my personal philosophy on PA and "clean" ss bass rigs, I like to have a large margin of power in excess of the speaker's thermal rating so that I can attenuate on the front end of the amp. There are large transient signals that almost always exceed an amp's maximum input voltage, even if it's only for a few milliseconds, and if you can turn down the amp's input and prevent that, your overall sound will be better and punchier and your amps will last longer.
