Ignore what others think is good....in particular in a car. I'd bet no one you run into will have a "good" system. If you want a reference I'd start with home audio. Of course the most cost effective home audio is a good set of headphones. I personally have some AKG cans and some custom molded in-the-ear monitors which both absolutely destroy ANY car I have ever heard. All that being said, it doesn't matter if headphones sound better or worse to you, what matters is what YOU like. The benefit of a headphone demo though is you will hear all the dynamics you might be missing which will let you of course then compare. Do beware, you are heading down a slippery slope of no satisfaction ever once you start to really analytically listen. While it is slippery it is also a great ride though so don't be scared.
I sold my PRS almost 5 years ago. The EQ on it is graphic and my memory says it is rather easy to find...more so than the crossover points. 16 bands of Left and 16 bands of right. Our interest is only in the bands that are not at zero.