How about a sealed box using the subs you already have?
I ran the rough equivalent of a FI Q18 in a 120L sealed box powered by 1.0-1.3 kW.
Would hardly break 130, but it sounded pretty clean.
In the grand scheme of things, that driver still isn't very good for SQ. Most of the drivers mentioned here won't be that great.
You'd want to start looking at something with an XBL^2 motor (Creative sound solutions sells some XBL^2 subs, I think Mach5 audio might as well, Mach5 underwent a name change recently: Looks like machs aren't, but non-metal former is interesting.), the LMS motor/coil (TC Sounds, say what you will about Thilo, the LMS 5400 is still king for clean output) or the lambda motor (Acoustic elegance run by a guy named John).
Those are a step in the right direction if you're after some obnoxious sound levels still. Though the lambdas tend to stop around 15 mm Xmax IIRC. Probably comparable to what you have now though. Except they probably have 5-10 dB more sensitivity. Reduces the load on the amp, and your car's electrical.
Otherwise, a Dayton reference series sub would be the ticket. Xmax is lower, sound quality is there, value is there as well.
But again, I can't see how you'd run a ported box and have anything close to a flat response curve. I'd have to see the chart to believe it. By flat I also mean balanced with the other speakers. Flat from lets say 30-16 000 Hz.
Model the subs you are considering with this program: http://www.linearteam.dk/default.aspx?pageid=winisd
If you wanted the excursion, this might be a cheap yet good sounding option: http://p10hifi.net/CSS/CSS-SDX12-data-220112.pdf http://creativesound.ca/details.php?...SDX12%20Single
http://www.aespeakers.com/drivers.php?driver_id=7
www.parts-express.com/pe/showdetl.cfm?partnumber=293-666
http://www.parts-express.com/pe/show...number=293-662
http://www.parts-express.com/pe/show...number=293-658
https://www.parts-express.com/pe/sho...number=295-466
https://www.parts-express.com/pe/sho...number=295-469
https://www.parts-express.com/pe/sho...number=295-512
http://www.istonline.ca/mach5_phallus.html
Good place to check for some data on drivers in different systems: http://www.data-bass.com/data?page=home
