Speaker setup in my ranger...suggestions?

[88ranger5speed]

I got two 3 inch jbl speakers in my dash from the 90's, didnt sound to good, so i put two pioneer 6 inch speakers behind the seats in the stock boxes i pulled from an xlt model. It's a single cab...but i wanted to put a sub between these speakers, but i have no idea what to put there, my uncle works at best buy so i could probably get a good deal on a set with a truck box...but i want a good sound for a decent price (under 200)