Sounds like a problem with the amps input stage (rcas) and if i read your post right u are piggybacking rca's from one amp to the next via rca outs from 1st amp to the second which is transferring the interference from the bad amp to the other. That can be repaired, shouldnt be too expensive, i have had that happen to me, costed 120.00 to fix, was quite awile back.

 

i'd say red means protect (JohnVroom) which could mean the impedance of the sub is too low or insufficient power, etc. my amp did this once to me and was acting all funky, i just unhooked the amp for a day, unhooked the sub, rehooked just the ground, power and remote turn on and made sure it powered on, then hooked up the sub and made sure it worked. if at anytime the red light comes on again (depending on what wires you have hooked up) that would be your problem area.