I had a mtx7500 dual coil 10- ~400 watt if I remimber, and a 1600w old old legacy running on one channel, turned half way down and it was actually good quality, JL is probably a bit better but you can get both for like $2-$250 new now + a box and wiring. I'm not saying you should go with either one just saying keep your options open. I hate seeing people blow a lot of money on just a sub setup when you arn't competing in anything. for 1200 You could do sub-amp, and replace all your componet speakers + amp.
I never had problems running a 2 channel bridged but I hear a monoblock is the best if you don't plan on running anything else off it in the future.
Goodluck, and I'm sure whatever you decide on will make the gti sound system sound 10 times betterm because it did in mine