Stereo guys, please help.

Stereo Guys I need your input:

What I am trying accomplish is this; I want to be able to hook up my MP3 player and have more control over the bass and sound of the system. I am not looking to replace peices of the system (besides the sub), just add to it. I don't want to spend a ton of cash on this project.

The system:

I have a upgraded stock system in my 22v. The system has 6 clarion component speakers in the cockpit and 2 Rockford Fosgate components on the tower plus a clarion sub. The system has 2 clarion 400.4 amps. I am not sure if all the speakers are coming off the amps or maybe a set of them are driven by the deck. I haven't looked into the system that closely.

What I am thinking:

I am thinking of getting and equilizer so I can input the mp3 to one input and the stereo to the other. This will provide the control I want over the sound and solve the MP3 player issues. Here is where things get a little hazy for me. If I want full control of the bass I am going to have to add an amp just for the bass correct? Are there any good free-air subs that I can replace the Clarion with? I'd like to use the 2 existing Clarion 400.4's for the cockpit and tower speakers and another amp just for the sub. Would another Clarion 400.4 work for driving a sub?

What's a good equilizer for this application?
Good sub?

Thanks for your help.