My buddy is getting a 2003 cadilac cts with the Bose system but wants a new system, he said he wants to spend between 600-700, he needs deck, amp, sub, speakers, but I can help him with everything else I'm just trying to get some opinions from you guys since you guys are usually good with this stuff, oh and he wants something that sounds good but can get loud so SQL
type r 12" $120 http://cgi.ebay.com/ALPINE-NEW-TYPE-R-12-SUBWOOFER-1500W-SWR-1242D_W0QQitemZ3304 15145743QQcmdZViewItemQQptZCar_Subwoofers_Enclosures?hash=item4cee47030f#ht_4210 wt_1167
you could also go with that type R 12" in a bandpass or ported box, and use a 5 channel amp or 3 channel amp solution, which would allow you to keep everything very simple for wiring and installation, and probably save a bit on total cost.
entirely depends on what order of bandpass enclosure it is, and how it's tuned. A decent 3rd order box can hit decently down to around 40Hz, and still gain about +6dB around that frequency. If you tuned it for 55-65Hz, you'd lose your bottom end but you'd be up +12dB.