I suppose you could try running your mids (and twetters right, you do have twetters along with the mids?) off the reciever; but only if its a good power reciever, because i know that some factory recievers only do 4-5watts--yuck. As for the 6x9's, i suppose a amp could help, but where are they mounted? More than likely the 6x9's are on the rear deck in "free-air" style (no enclosure), so the bass response may not be "quality". The amp would boost the 6x9's midrange and twetters, but bass wont improve much without an enclosure, and more than lickly neiather will your fronts (thats where the sub comes in to play). Try improving your front midrang speackers enclosure; where are they mounted? Door locations are somewhat sealed enclosures, but they are resonant. Baffles, and sound-deadening materials help reduce resonant and lower road-noise that interfers with your speacker. The enclosure will affect your mid-range, so i would try improving the enclosure (baffles are cheap and easy to install). Personally, i prefer the rear speacker (6x9) to be used as "fill", and have a subwoofer do the lower frequencys, but if your expecting the 6x9's to do bass without an enclosure then i wish you luck?
If you want to get the "most" out of your front midrang, then i personally would get an amp (that is if the front speackers are good in the first place). Amplifiers improve the sound from the midrange, and make the twetters shimmer more lifelike, but they basically have to be good speackers in the first place; amps on front speackers are only "necessary" if you come into distortion problems when you turn it up, or if thers no "loudness/fullfillness", so i would try running it off the reciever first, and try to improve the midranges enclosure...then see if you want more power.
i don't know, maybe someone else can give better advice, or share there own experinces. I've never had a car with a good front speacker locasion; my car has the "unenspiring" dash mount small wimpy 3.5" speackers, so i cant really do much and cutting a hole in my door is a risk i dont want to tack.
