As said above you can certainly accomplish your goal of adding a amp or two and a sub or more while keeping the factory heady unit. There are many ways to skin a cat...
Basically the simplest of ways is to make sure the amplifier you are using has "High Level" or "Speaker Level" inputs. This will require you splicing into the factory speaker wires to a particular speaker to tap into the audio signal and feed it to the amplifier through its supplied adapter harness.
The second cheapest method would be to purchase a Line-Out-Converter (LOC) which will splice into the speaker wire and convert it to an RCA output which you could wire to the amplifier (Low level input). These can range in price greatly, basically for a simple splice/converter box to a splice and boosted signal box.
The third option and most expensive would be getting an OEM integration box or processor such as a JL Audo Cleansweep or similar processor. These are basically complicated processors which allow you to keep the asthetics and interface of the OEM head unit but strip any audio processing and then control the audio signal giving you full control of the shape of the signal sent to the amp(s)
