LOL, a line out converter thingy? JL are poopy?
I think my 2 JL 10w6v2's are great! and here's a few people who've heard my car and were impressed. That being said you normally get what you pay for, but a few local fortes own this:
JL Audio CP112-W0v2 Subwoofer systems
and they seem to be quite happy =)
anyways, if your amp has a high level input or you buy a line converter, you can splice in to one of your rear channels. The way the local installer has done koups is they've spliced into the cable run that goes along the driver side of the car uner the trim (there's a bundle of wires and one pair heads to the rear left speaker.
I agree with the bestbuy comment.. there are some decent installers, but most are crap. Your best bet is a reputable audio installer.