I'm not sure how reliable the source is, but according to the following site, the stock head units for the KJs are from 62-66 watts.
http://www.jeepworld.com/accessories/liberty/stereo.htm
Unfortunatly they only seem to be posting the peak number and not the RMS numbers. If you're planning on speaker upgrades, and want to keep the factory head unit, I'd recommend an amp so that the speakers are adiquitely powered. The stock power (16.5w per channel) is not going to hit the sweet spot on any decent pair of speakers available in the aftermarket.