I'm not sure what's wrong with my rear Speaker input preamp on my pioneer deck. Whenenver I plug any RCA cable it starts making loud unwanted noise. I have 2 sets of preamps on my HU . I'm using my front to power a 4 channel amp ( I'm only powering my front components) . I'm using the rear preamp for my mono viper 1200.1 amp. One last question what can be wrong wth my rear preamp that makes so my noise whenever i turned it up. My gain is set correctly so i know that's not the problem, and I try using different RCA cables so thats not it either. Both of my amps are well grounded . My front speaker preamps work fine Is just my rear one that make a lot of noise specially when i turned it up i can hear engine wine and a pop sound
try putting your viper amp on the front preamp. this will let you know if it's in the amp or the head unit. Sounds to me likt it's the hu, but try that, if you no longer hear the noise than thats the problem