either go all stock on the infinity system, or all aftermarket. don't mix/match.
first, the impedence is different. if standard speakers are 4, then the infinity's are 2. or the other way around...I can never remember, I'm not really a stereo guy.
but, the amp is matched to the speakers, and it's meant to work as a set.
my dakota had an aftermarket head unit, kicker drivers, the stock infinity tweeters, and the stock amp bypassed.
sounded like crap. the highs out of the infinity tweeters were so high, it would make your ears bleed, and no bass from the kickers.
I just put in a set of Alpine component set speakers, and already, it sounds better. but still not the bass I want.
but, I'm happy because I know that for the bass I want, I need a sub and amp. not after all out volume, so I think an 8" sub with a "mild" amp would work fine. just gotta go find them and buy them and install them!