if your goin to purchase an amp and sub for some decent bass u need to match it at the front with a pair of decent splits, as FTOluv has said before the splits are probably the most important speakers in your car, bass will pretty much travel through anything but treble is very position dependent
check out
http://www.ftoaustralia.com/modules.php ... sc&start=0
and http://www.ftoaustralia.com/modules.php ... pic&t=2287 for some more bedtime reading

oh and IMO u won't b needing the 6x9's after installin an amp and sub, u can leave them there for filler but if your gunna do that u'd b better off installin some good 4 ways, that way your only gettin bass from your primary souce, the sub, and not mixin it with the 6x9's