Underpowering subs is the #1 way of blowing them. When you force the amp to continuously drive its full current, you are likely to get long spikes of DC power -- which is not good for an AC device (like a speaker).
I'm sure you've noticed distortion and *probably* some sustained thump that's not supposed to happen.
Always drive for having 10-20% over a speakers' RMS rating. You'll have much more crisp bass hits and sounds that actually match the music.
Ok, I've heard that you can elliminate distortion (at higher volume levels) from a head unit that powers the speakers off it's internal amplifier circuitry by installing an amplifier (connected to the head unit's pre-amps, of course). Is this true? Something about the fact that the amps power is "cleaner" at the same(or higher) power level.
Well, not sure exactly what a "ported box" is (I assume it is an enclosure for the speakers with some type of vent/port??) - but thanks for the answer davids2004. (look - I registered!) =O