Alright, I have a 2009 1500 Crew Cab. Standard stereo RES HU.

I want to add an amp and sub (which I have.) I will then change out my door speakers (will purchase as I install sub.) I will then, in time, get a new head unit (because the one/ones I want are too expensive right now, and the RES has features like HD radio, mp3-CD, and also the free satellite for 1/2 a year.)

What PAC will work with this progression?

Thanks,
Wade