my brother uses a kenwood amp to power his componet tweaters and 6.5s and they sound really clear both are also kenwood. i use kenwood 6x9 wich produce better highs than my 4x6 and nasty lows.
if you want clear highs i would have to recomend either a set of kenwood tweaters and/or infinity 4x6.
did u say u had 4 6x9s in the back...wanna trade cars...i would kill to be able to fit that many speakers. i could probably waste at least $5k on a system for that car.
if you want u could get a pair of either rockford or infinity 6x9s (both produce nasty lows) and amp them with at least 200w with a low pass. instead of putting a second pair of 6x9s u could put in either 6.5s or 5.25s with a high pass so u get good mids out of it.
same setup could go for the doors. set it up so u get clear, defined mids and let a pair of tweaters pull out most of the highs.
im not sure how your deck is for watts per channel but looking at a crutchfield mag right now i can say a decent audiobahn 105w x 4 amp would run u about $500. you would want a separate amp for the 6x9 (id recomend either rockford or alpine)...im rambling and probably making no sense...sorry