^vehicles simply do not boost the 30-40 range there are going to be many resonances and cancelations. The response is going to be no where near that graph. Just an obstacle to battle.
Low bl sub in a small box? Crazy
And there's no use running a re se only on 400 watts in a smaller than optimum enclosure. I will agree that the enclosure is more important than the woofer.
My sugestions with that small of a box requirement .75~ cu ft you will be limited to 8" drivers. With that I would recommend sundowner E series or SA series. There are determining factors to how subs preform in smaller or larger boxes.
If you can go a little larger 1-2 net cubic ft (2-2.5 gross volume) then I would recommend a 10. I just don't see replacing a 10 in a .75 box working well at all no matter the manufacture.
Budget 2-3k
I already have selected $1500 worth of just audio then I need to buy wood and finish designing my enclosures. I'm sure my front/center/sub stage will be quite substantial for any room I put it in.
Theater would go in a finished basement with theater seating.