Audio Issues

Pioneer Premier deck hooked up to 4 speakers (wired for 6)
 
When I try to turn the volume on my deck to 20 or above (it maxes @ 30) I get some serious crackling/popping/distortion coming through my speakers. I've noticed that it does this through all 4 or when I fade competely to the rear 2, but when I fade completely to the front 2 there is no problem.........Any ideas what's going on here?

are the rear speakers factory?if so its probally drowning the speakers in power or they are dry rotted.

Unfortunately, yes. Why would I be hearing the noise coming from out of all 4 though?

Anybody think that the internal amp on the deck could be messed up, and that's what is causing this?