All good points here. My understanding in getting bass up-front is first and foremost the fundamentals. Before working on the crossover points, TA and EQs and processors, I would make sure the following is solved:
1) Good mounting of your midbass drivers, so that you'll get good midbass that will help pull your subbass further front.
2) Sufficient power to your midbass and like what wengweng pointed out, a good amplifier for the subs too. Clean power and headroom is what we're seeking. Not so much of power ratings.
3) Solve all rattling problems from the rear, including C-pillars and rear parcel shelf.
4) Make sure you open a hole somewhere at the rear shelves to allow the bass to get into the cabin.
5) Make sure that bloody subwoofer enclosure is not leaking. If not you'll be more prone to getting mechanical noises from the subwoofer driver.
and finally,
6) If you can afford it, a good and fast sub that can keep up with the front mids.
