the question is should i allow the deck to run its filters along with the amp or should i turn off the filters on the deck and allow only the amp to control the frequency range being out put to the speakers/sub.... i have a pioneer DEH-P7800 a rockford fosgate 450.4 amp its powering two rear speakers jbl louds 6 1/2s and other two chanels are bridged powering a sub..

 

You only need one... I'm using the ones on my amps.

 

Just use one, but it never hurts to see which sounds best. If you have both on you'll have a very sharp roll off where the second filter kicks in.