good info on this site about all things car audio, this article covers too little power to speakers...
http://www.bcae1.com/2ltlpwr.htm Underpowering speakers is ok as long as you don't try to turn the volume up to compensate and damage them with severe clipping.
From the article:
Note:
If your speakers are capable of handling significantly more than your amplifier can produce, driving them with a clipped signal will not likely hurt them.
If the speakers can handle 3 or 4 times the power that your amplifier can produce, there's virtually no way to damage your speakers (no matter how clipped the signal is).
If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a clipped signal for extended periods of time may cause speaker damage and/or premature failure.
If your speakers are rated for the same power handling as your amplifier is capable of producing cleanly, driving them with a square wave signal for extended periods of time will likely cause speaker damage.