audio quality

Alright, I'm totally uneducated when it comes to audio in cars or audio in general. My speakers are starting to go and i still have the factory head unit in my car. As I am finally tired of the stock stereo and speakers I want to upgrade to a quality setup. No super bass stuff just quality...

I am a bit of a newbie when it comes to all things audio, and ultimately i decided to replace my shaker500 deck with a Jensen uv8040, my audio quality has seemed to have taken a bit of a dive, my bass isnt coming through very well at all and when i try to turn it up the quality is just a mess...