For rock you really want a solid front stage, spend some money on some good 2-way components and a 4-channel amp which you can bridge to them and run some decent power, maybe 200wrms per side up front so you have lots of headroom.
6x9's are great if you don't have a sub but if you do have one all they tend to do is drag the soundstage to the rear... when you go to a concert, you don't face away from the band.
Subs, flick them on Trademe and get a single good quality one in a good box. You'll be amazed what a difference it makes.