I've replaced with Kenwood dualmag's in the original positions in the front, mainly for treble.
They sound good quality - wise, (much better than the originals!) but they aren't designed for bass.
I then made up some MDF 'pods' to house 6.5 inch dualmag's, which i put in the bottom left of the front doors mainly for mid-bass. I'm running both sets of those speakers off of my Sony headunit.
Then you could put 6x9's in the rear shelf or a bass box in the boot to produce the bass you need, running from a decent amp. All in all it creates a simple, well balanced system.
Another tip is to make sure any speakers you put in are well secured to the surface, as this affects bass and sound quality greatly.