amp question

I have 2 infinity 6.5's in front and 2 alpine 6x9 in the rear.  When I turn my volume up the treble sounds awesome but the bass distorts once it gets pretty loud.  I was wondering if putting my speakers on an amp instead of the head unit (18w rms per channel) would let me go louder without the bass distorting, or if that its just the speakers limit?

There may be a slight increase in bass response with an amp, but you wil mostly be looking at an overall volume increase. In your situation a sub will play the biggest role in evening out your system. By your question and description it sounds like a sub is lacking from your system.

I disagree with DWarren there - you're having trouble with bass b/c your hu's amp doesn't have the balls to drive lots of power required to make the speakers do lower frequencies.  An amp will make things louder per se, but now the speakers will have the proper juice to reproduce bass.  So yes, an amp will help big time.  So will a sub, but that's another question.