Pull out your deck and check all wiring (did you use the the ground wire from the radio wire harness? if so, make a new ground from the chassis of the deck to ground) I have seen recently where someone swapped out a deck and replaced it with another one... they connected all the coloured wires together **except for the black ground wire** His subs would play until he turned it up louder and then the subs would lower in volume as soon as he reached a certain volume.