There's nothing wrong with underpowering subs. Keep what you have.
Just be sure to set the gain on the amp right.
The purpose of the gain is to match the signal volts RMS coming from the source (CD player, etc.) to the input of the amp for correct power matching and to prevent clipping.
Here is a guide that will help you set the gain correctly http://www.box.net/shared/uchv4dbk88
You'll need a multi-meter (AC voltmeter, $15 from Radio$hack), Microsoft Excel and a way to burn an audio CD from an MP3.
If you don't have Excel, use this link http://www.box.net/shared/nkkw1dhk4g
See my site for more info http://spkrbox1.spaces.live.com
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Some more unknown truths:
I can drive speakers with a 100% clipped square wave signal all day long with no problems as long as the thermal and mechanical limits of the speaker are not exceeded.
I can feed a speaker 100% distortion all day long with no damage as long as the thermal and mechanical limits of the speaker are not exceeded.
I can exceed the thermal and/or mechanical limits of a speaker and watch it fail in short order.
These are electrical and physical truths and anything else is a myth.