Just wondered why you're going for JBL speakers? As with getting a JL amp, they'll be letting it down a bit! Some JL, Rainbow, Focal, Boston acoutics, Hertz or DLS components will be much better.
Like said a sub will be much better than 6x9's. A 10" will be fine if you get decent power to it from an amp. One will be fine, but two even better, but of course more money!
I've got a brand new Phoenix gold 500w mono amp for sale which is ideal for a sub if yo're interested?! £110 cheaper than at Car audio security!