I wish i knew more about audio. I definitely don't want to get rid of my stock head unit. While we're on adding speakers, how hard would it be to install two 10's with a 2-channel amp? Can the stock head unit handle it?
Once again, yes, but you're going to need a set of RCA pre-outs to run the amp. Which means you're going to need to buy an interface to provide you with those and wire it up into the existing harness. Something like this.
http://www.crutchfield.com/p_161LC2I/AudioControl-LC2i.html?tp=61671
Also if you haven't bought the amp yet, don't buy a two channel for subs. Buy mono. You want the same signal going to all your subs at the same time.