take some pictures of how you have the amp wired up... take some pictures of how you have the HU wired up "rca's hooked up etc.." check to see if you have sub/rear control on the HU make sure it is on... Take a picture of the SUBS and the BOX... Also dvc? svc? what ohm... what amp? etc..
I've got 2 powerbass XL84's in my brothers truck. .7 cu per box. on 150watts to each sub... hits harder than the 1 JL12w3v4 oh 500watts ... just got to make sure you wire everything up correctly with the right box etc.. but like i mentioned and someone else mentioned take pictures of everything I asked for and we would be able to HELP alot MORE than atm with what you have told us