the stock 6" subs are 2 ohm speakers at least mine are.
I took out the sub capacitors ont hem, and put a crossover on them, with some tweeters I put in the dash location along with the original dash speakers, and they now put out voice, from the doors. This has to me, made the car sound better, because all the voice is no longer coming from the rear speakers only.
For radio's, most new radios will have front, rear and sub pre outs, if they are higher end models, and some come with built in eq for certain frequncies, it depends on the brand and model of radio.
If the radio you have has only one preout, you might want to look into an eq, to run, so you have front, rear, and sub outputs on the eq to go to the amp or amps.
A Good 4 channel Iw ould think should be able to handle the speakers in the car, if it is stable at 2 ohms or 1 ohm, in either stereo or mono, preferably stereo I would think for the interior speakers.
If you are adding a subwoofer for some added kick, you can have a custom dual 8" subwoofer housing made to fit the first aid box, which should add bass without being really boomy.
My setup, has the following done...
Factory interior speakers all around. Tweeters added to the dash location glued to the bottom of the dash speaker grill. Factory dash speakers are still there and hooked up. extra tweeters are hooked up to a crossover, and door subs were modified to hook up to the crossover too, to make them midrange. Rear speakers havent been modified at all, and dash speakers were not modified either. They are wired up to the radio's built in amp, the door subs, front dash speakers, and tweeters are ont he front channel. the rear speakers are on the rear channel. I then have a 10" sub, ported throught he first aid kit, with a 450w (at 14.4 volts, but I'm not running a capacitor to bring it up to 14.4v, so its running at 12v give or take a little.) I estimate that the 450 watts, at 12 volts, is lets say 200-250, maybe a little more, maybe a little less.
My subwoofer is an 8 ohm sybwoofer, that I bought several years ago. It hits nice.
I ahve the bass ont he radio turned down -2 levels from the middle, and the treble is set to +1 to +3 depending ont he music I am listening too. The sub is set to around 80-120 frequency, and amp is set to just over half gain. It hits nicely, provides the vibration formt he bass, and doesn't to me, sound too boomy at all. When I put in boomy music with good bass, liek some hip hop etc, it can thump the bass nicely but still not be too boomy to me. With rock, the bass sounds good, not too much, but maybe just a little less than I would like, but I can usually turnt he radio's bass level up from -2 to -1, then it sounds better on some songs.
The car is plenty loud liek this. I will be adding a 4 channel 400w amp to power the interior speakers, and changing the headunit for a pioneer 840, because I am gettign the radio form my friend, who I traded some computer parts with for the radio and amp.
Later I will upgrade the speakers for sure, but for now the factory ones, sound good enough.
I had originally planned on building a computer into my car, but decided I didn't need to have one. The new radio, will play mp3's off a cdr. with 150 + songs on a disc, who needs a cd changer?
My factory speakers have different capacitors on them depending on the purpose of the speaker. the only ones I have modified were the door subs, so they would be a midbass, by adding a cross over instead of capping ti as a midbass. The crossover has made them sound better.
Good luck on your upgrade. let us know when you are done.
Alon