factory sound processors ?
Video: JBL MS-2 Car Audio Optimizer
The JBL MS-2 is a portable digital signal processor that improves the sound in your car when you use a portable player like an iPod. It does not require permanent installation, so you can take it from car to car. It'll clean up your factory sound, even "premium" amplified systems. And it also works with aftermarket systems, including those with subs.
In this video, we hook up an MS-2, run through a setup, and then switch back and forth between the factory stereo and the optimized audio from the MS-2. Put in some earbuds or don a pair of headphones to best hear the difference.