Upgrading base stereo (keeping oem head unit)

peleincubus

So I have read over some past threads, but they typically deal with Bose systems.

Im trying to gauge how feesible it would be to put for example hertz component speakers and sub in the spare tire enclosure along with an amp while keeping the stock head unit.

Is there something you can use to bypass the head unit and connect the amp to something else? So you can use the full potential of the hertz speakers and sub?

It has been a long time but the higher the volt the better the sound quality correct.

I guess for the most part I want to have a quality sound system but for it to look oem and completely hide the amp etc.