The JBL setup with the sub is the way to go if you want the best of both worlds in terms of combining studio monitors and a bass amp. The sub will give you more accurate low end on your mixes and makes practicing bass more gratifying since you're moving more air. I used to practice through my nearfields (I have some excellent 8" Tannoys) and while it was totally acceptable it wasn't nearly as fun as playing through a bass amp that moved more air.
Thanks for the tip. I suppose that a sub could very easily be added after