Reply to Need a sub?

I just got a set of jbl jrx 115's for my live setup. They are ok but I don't feel like they have the proper bass. Maybe 50 hz and below is weak or non-existant.

My question. Do you believe I could get a decent sub for 500 or less? The jbl's are passive and I have a QSC rmx 850 powering them. I don't have a mixer. Just my laptop, controller and a 4 ch sound card.

What would I REALLY need to get to have some decent subs jumping off? Any suggestions would be appreciated.