whats your price range in this?
ill tell you my setup
I got 2-12" W3V2 JL subs being pushed by a JL 1000W mono class d amp running at 2 ohms ported
Front speakers are JL components 6 1/2 w/ tweaters. Components are like a crossover so it seperates the highs, mids, and lows.
Back speakers are JL 6x9. These are cheap walmart speakers
for like $50
All my speakers are being pushed by a kenwood 400W 4 channel amp
Got a Pioneer Double-din 50x4. Built in EQ, navigation, bluetooth, all that good stuff
If you say you want to keep your old factory sub box you are going to have to find out the specs of that box and match it to a sub with similar specs. A good cheap sub is that i would recommend is a Kicker Comp Cvr. Comes in all sizes depending on how big your factory box is. The amp you want is up to you. I always say the more watts the better because you can always go bigger on the sub. 6 1/2 speakers are good enough. I would check out component speakers though. Listen to the difference especially with the tweaters. If you want it to be loud and CLEAR then i wold recommend a 4 channel amp. It really helps a lot.
If you are planing to install this on your own make sure you take out the factory speakers first and size match it to the aftermarket ones you buy. I once bought 6 1/2 kenwoods and tried to put it in my front doors. Came to find out the magnets on these aftermarket speakers were bigger so they got in the way of my power windows. I couldnt roll the windows all the way down cause it would hit the speakers lol
Let me know if you need any more help or suggestions