What's wrong with the amp in a half decent head unit?
Amps = Quite a lot of f**kin about
Cable ALLLLL the way to the boot and then ALLLLL the way back. That's not good for audio as it is!
What would you go spending on an amp to drive 4 speakers then?
to be honest you can spend anywhere between £50-100 and get a pretty decent amp for the work you're looking for. If you use an amp they are much better as they will control the sound going to the speakers a lot better and as Russ said they will deliver more power to the speakers, helping to get the full capability out of the speakers. You can run speakers from a HU dont get me wrong, but you will be more at risk of blowing the speakers if you try and to the volume up and the sound normally will not be as clear.
If I'm wrong some-one can correct me I'm sure but I ran speakers without an amp in my 1st car and they blew within a few weeks, I amped them on the next set and they sounded much clearer and never died