I agree that 30 is too low, unless you use like a 6 db slope but such near-field conditions to the subs have to be considered as well.

I have a regular cab full-size 85 Chevy 1/2-ton and I use a ported 8" sub sitting on the tranny hump. I cross it at 50 Hz with 24 db slope while sitting still or at 80Hz when driving. Anything above that and its too much information through the sub. Being so close to it, it doesn't quite blend as smoothly with the rest of the speakers if it was sitting way back in the trunk. I too use a 7" midbass (B&W) and I have mine crossed at 50 high-pass 12 db and about 150 at 24db low pass. My 4's come on at 200. I had to put a gap in there since my truck seems to have a nasty resonance around 200 from the 7s.