JL and JBL are different manufacturers. I believe JL are generally higher-end (i.e. more expensive) and JBL aren't, but I'm sure there's some grey areas..
I'm not really the guy to ask about subs and quality... I Just grabbed a Halfrauds Mutant Sub+Mutant Amp+Box+wiring deal (£100) for mine (I was getting annoyed with lack of bass really not doing Prodigy justice). I'm not the type to Want really really really silly loud so people can hear you coming from miles away, but some tracks require a bit of oomph...

I'm perfectly happy with it. I'm sure if I was really paying attention I could tell the difference between mine and a higher quality one, but at the end of the day I'm listening to tunes whilst driving, so my full attention is hardly ever on the music - usually on not crashing into cars/walls/people/etc...

I think unless you're budget is really high you're better off spending the money on a decent set of components for the front, so your high/mid stuff is all top notch. That's where you'll really notice sound quality differences IMO. Then whatever's left over for the sub.
This is just my opinion of course - others may disagree (hell, I might disagree myself in the future!)...