question regarding tahoe audio system

i have a 1999 4 door tahoe, and im going to be replacing all the speakers real soon and the head unit, for the front i bought some kicker components to replace the factory speakers, for the rear roof speakers i have jbl 6x9s that are going in, for the rear doors i was thinking of putting 6 1/2 subwoofers instead of regular speakers, since they are run by the factory amp, i think they would produce just the right amoount of bass that im looking for, has anyone done this? does it sound like a good idea/ or im i better off just putting a single 12" sub in the back, any comments or ideas would be highly appreciated it, thank you