I've had my bit Ten in my car since last September and I've been tuning regularly all of this time. It's taken a while for me to really nail down what each frequency spectrum offers to the music but i'm getting really good at that now. Part of the problem is that if you have peaks somewhere, that will influence the entire spectrum. For instance - too much in the 250 and 315Hz area adds too much mud and thickens up everything making it really hard to really hear the contribution of the 2kHz on up areas. Now everything is very balanced and sounding even better than my home stereo towers.
One of the things that i have learned through this long process, is be careful making deep cuts. You can get caught up in a downward cycle. Lets say you have a null in the 400 to 630 region so these should be cut minimally. Well if you have them cut too much it makes everything thin. In an effort to fix this, you might go and cut the upper mid ranges more and before you know it you've added a couple extra db's' of cutting in the 2 to 4kHz region to fix a problem with the improperly set 400 to 630 range. You do this and then the 800 to 1.6kHz regions seem too hot so you cut them too. Jeez, it gets crazy after a while. All of the while maybe, you cut 400 to 630Hz too much in the first place because the 200Hz to 315Hz area was too hot in the first place.
Anyways after all of my tuning I have learned that less cutting sounds much more natural than broad deep cutting. For the most part, I only have deep cuts in the 200 to 315Hz areas, the 3kHz on up area on my far woofer, and in the 8kHz to 20kHz area on my tweeters (I don't like a thin overly sibilant sound).