loudest/warmest sound in an impreza?

Howdy ho 12volters! 

I really need to ditch these flaccid, high-impedence CDT 12's I'm running now and drop in something that can standup to my front stage.  Now that I'm finally on winter break with xmas in the rearview, I can get some R&R with a table saw and some MDF! :D 

What is the loudest (preferably JL) thing I can stuff into the back of an Impreza sedan and wire to a JL 1800?

I'm thinking my trunk is good for a maximum of about 2.5 ~ 3cft net internal.  More than that will not actually go into the restrictive trunk opening, even if the volume would technically fit.

So I have 5.25's all around in the cabin - I can get about 125hz out of them while retaining any respectable output.  Hence my reservations about grabbing a pair of 4-ohm 12w3's and being done with it - 12's are leaving me with less than zero mid-bass and I'm really trying for something a bit warmer. 

How about underseat 8w3's?  Three 8w3's in the trunk? 

Could I do something overly complex like underseat mid-woofers and a low-ported 12, 13, or 15 and a passive xover - I could grab a pair of A/B channels to run this mid-woofer range if need-be but I'm not entirely shure of a clean way to grab that particular signal sweep.. (uh- heamph)? 

Not overly concerned about cost here, but I cannot change my HU, amps, or cabin speakers.

How about this sub: https://www.diycable.com/main/product_info.php?cPath=24_93&products_id=653

It uses an Xbl^2 motor so inductance will be lower providing a better high frequency extention.  It also means much lower distortion giving you clean, accurate bass.

Hmm.

Given my aforementioned space considerations contrasted with the listed application notes at Steven's link, would these really be appropriate?  Even in a heavily stuffed box, I'd have slightly <1.5 cft per woofer... I'm concerned they'd sound a bit choked.

In a sealed box, these look about like anything else modeled in WinISD, so I'm really looking for musicality and tonality here.  The application notes do stress these strengths..

I really wanna switch gears from my usual goals/methods for an install (dead flat curve from a ported box). 

I'm so sick of this flat sort of subterranean sound I have now I can't even tell ya'll.

I guess what I'm saying is that I really wanna try something besides *more* 12's.  I've always had 12's. 

Ya'll really think I shouldn't?

What about doing a single 12 ported?  1.5 cubic feet for a ported enclosure is pretty small for any speaker.

You can definitely get a smaller coned driver, but you may give up some low frequency extention.