the internal sound shaping and parametric EQ is great. The only time you ever really need an EQ, is when you need to compensate for a poor choise in speakers, or a bad acoustic environment. It's a compensation tool, and not something you want really just to mess with sound curves all of the time. Once the system is tuned correctly, you should never need to mess with an EQ.
Haha, im sure my car is a bad environment, its a 1991 isuzu trooper. I did put some dynamat in though. I have the 9815 and am in the process of adding a sub and amp. After that I would like to use the HU to adjust all the crossovers so the front and rear speakers only get the signals they need (Im not sure but hopefully that will utilize the power of the internal amp better). But as far as using the equalizer im in the dark. How do you know which frequencies to adjust, and to what extent?