yesterday, i had a huge post that answered everything, and then my computer went haywire...
btw, welcome to the forums!
for some of the points you made: most factory audio systems these days are starting to use subs and tweeters for better quality audio, so a sub can definately help.
also, the equipment you have should work-- it is cheaper than the alternative
these forums are primarily for installing computers in cars, so audio questions might be answered faster/better/differently on a dedicated audio forum(there is a sticky at the top of this forum area if your not sure where to start)
i have a similar audio setup, so will comment a little:
i would recommend a setup similar to what i have-- using a active setup, with a sub, and component speakers. (i have a convertible, and like loud, clear music)
the reason for these: the sub will allow you to play loud without stressing your main speakers.
a subs output is only limited by the amount you want(using the gain)-- you can tune it so there is almost nothing, or so that is all you hear...
the active crossover will allow you to ditch the crummy passive crossovers on the components, and will allow you to tweak the level of the tweeters, mids, and subs to your prefered level, and will also allow more output, by allowing you to change the crossover point.
that is good enough for now, if you have any questions about it, let me know