problems running 2 amps

i currently own 2 12" pioneer premier ts-3004spl's and i just recieved my second kenwood excelon 1600 watt mono-block kac-x812d amplifier. in my install i hooked each of the amps to each of the subs and my plan was to run rca's from the output on the one amp and into the input on the other one. i did this and when i plug in the second amp the sound quality of the subs goes horrible. it is louder and sounds much better when i only have one amp hooked up to 1 sub. i own a panasonic c7103u head unit that has a 5 volt pre-amp output so there should be plenty of voltage for each of the amps to run off of. i also tried using two y-splitters at the deck and ran a seperate pair of rca's to each and that didnt work either. it is also possible to run a s-video cable between amps and that didnt work either. Any idea's?

One of your woofers is wired backwards.  Or if the amps have a   0-180 switch on them they both need to be in the same position.