Weird amp issue.
 

Amp is the Soundtream TN 2.600D.

Buddy recently installed one in his boat. While not the best amp he was just looking for something with a very small footprint to power (4) 6.5 Wetsounds 65 speakers. The amp powers up and goes on. The issue is the minute he connects the RCA cables to the input , the amp shuts down. He's tried 2 different sets of cables ......same results. Also tried a different amp, no issue. Any idea what is going on here? Simply bad amp?

Sounds like just a faulty amp, unless the gain is too high and it's clipping - resulting in no sound coming from the speakers. But if the amp is completely shutting off its probably just faulty

tell him to measure the voltage between outer ring on the RCA and the ground on the amp. I suspect you may have a floating voltage issue. The deck and EQ if it exists should be grounded to the same potential as the amp.

Take the cover off the amp and see if anything looks burned. I've had two amps burn up their mosfets and they would shut down like yours. They can actually be replaced.