Overlapping Crossover Points

The acoustic environment in each car with each varying system is different.  Sometimes you will get the best response with some overlap.  The tweaking you have been doing is telling of that.  With most of the rock music, there is very little actual subsonic frequency response in the recording.  You feel like the subs are largely going to waste.  But that is just the nature of the type of music.  Ideally, your midbass/mid woofers would be the ones that will pick up that lower midbass response and give you all the kick you want to feel.

In your case, because of the installation or the type of woofers, whatever it is...the front midbass isn't up to your expectations, adding frequency response to the sub is a bandaid solution.  But there again, every car is different.  If you're not hearing excessive boominess in the 80 to 100 Hz range, then it would be fine to tune the subs up to that frequency range, as long as the stereo reproduction is not being blurred.  The snap of the drums should be felt and heard in stereo from the front soundstage, and there should be a feeling of spatial reality in many of the better recordings.  If bringing the sub up is blurring that spatial reality, you should work on improving the front bass by adding more damping, better woofers, better positioning or whatever it takes.