After #4 is completed on the my current components.... I'd rather not add another layer of complexity until I'm comfortable dealing with what I already got
Besides, I feel there is clean/strong output down to ~37hz from the ZR800s.... so only really missing 1 octave.
But, if you want to know the options I'm considering in that dept:
1. Small sealed enclosure 10/12 in passenger footwell like Jason B's car. Perhaps Oncore SM10/12 or Si MkV.
2. Small sealed enclosure 2 x 10 under rear deck with cones facing cabin like your car
3. IB setup on rear deck.
Need to listen to more cars to get a better idea of what I like best.