Ok I am working on a new headunit for my xfire.

I want more sub control.
I am looking at a head unit with a sub. pre out, and sub control
Can I just get a two channel amp and hook it up, running the sub pre out to it then running the powered wires directly to the factory subs, Of course I will need to disconnect the wires from the facctory amp to the subs.
I hope I am making sence, It is hard to type what I am trying to say.
What amp should I get?
Any better suggestions? That won't cost tons of $$$
Thanks