^ does that really make a big difference? (playing the 5" subs less then an octave) it looks more like just to bump up 80hz then anything imo. which could be achieved by tuning or using the correct x-over topology with the main subs.

 

i had them lowwer and yes it does help

 

i know this is kind of stupid and you probably have done it but is the car sound deadend have you taken care of the rattles and what have you.
as stated previously, the sub frequencies are non diractonal and really shoud be treated as such, i would fear that placing subs in the dash of that car is going to cause more noise from pannels ratteling. i do understand what your trying to achieve, but i dont think your going to be happy with the expence and the lack of low frequency......... a single polyglass v2 13" would be a much better choice.