I have three amps running my subs and tweeters. my bass hits to hard and with my alpine doesnt have enough power to hear my tweeters. youan have only one amp if you your bass isnt hitting hard so you can still ear the words. I hope this helps.
if you just have a pair or 2 of some run-of-the-mill coaxials, and that's all you've ever really heard, then a basic head unit will have enough power to get them sounding pretty well.
But as Cgarza said - once you add a sub you need power to the other speakers to bring them up to an equal level. Plus, once you hear a nice, amplified set of front speakers you'll realize all the detail and clarity you were missing from your basic coaxials getting 20w.
If you never turn it up -- then an amp probably wouldn't do much for you.