So.. a simple check on the RCAs would be that (if you have the HU pulled to get to the backside of the HU, #1 Check make sure you are on S or SW L+R OUT on back of the HU, or Rear if you have it set up that way. Take one RCA, white and plug into Left on HU, and L on Amp, if works, Great! Then remove and place R/Red on L/White on HU and same on Amp in and try same with other ch as a single RCA Out from HU to single IN to amp on both RCAs, If both work R/and White at a single run from HU to Amp , the RCAs are fine. Its going to be the HU. It has an Internal issue recognizing a dual input on sub ch from the HU board