Look for a sub that has dual 4ohm voice coils. That way you can wire them in parallel to 2ohms at the amp. Get one that is rated for 3-500watts RMS. Make sure it performs well in a small sealed box.
The 2channel amp will work fine with those front speakers. If you want, get some Infinity Reference speakers that are 2ohms -they'll sound better than Kicker and they'll get the full '150 watts' from the amp.
You can still power the OEM rear speakers with the rear outputs from the stereo.
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2013 Kia Rio -90a alternator
DDX470HD GTO14001 GTO1014D (x3)
Big3 in 1/0G
1/0G to GTO14001
Good call on the infinity 2ohm speakers. same with the JBL 2 ohm. FYI both JBL and infinity are owned by the company Harman/Kardon a.k.a. Harman interhational industries. JBL and infinity products are, for that reason manufactured very simularly. I have not compared those 2 pairs of speakers side by side, but i would bet that they have almost identical specs, and sound very simular, if not identical. If you look at infinity's show cars from the years after they quit producing the beta series amps, and before they started their referance series amps, they used all JBL amps. the reason for this is, that they are the same company. here is a link that you can verify what i am saying
https://en.wikipedia.org/wiki/Harman_International_Industries-------------
If your cousin is such a good installer, and he will install anything for a 6 pack; why are you talking to me?