I think you called it. The amp wasn't showing any changes in signal lighting when shutting off. It just went silent, so I couldn't try your suggestion, BUT, after trouble shooting everything else, with the feedback thing in mind, I freed up a balanced output from the mixer and used that (to the balanced inputs of the amp) and VOILA! everything works fine.
Did the lighted ring around the level control show the presence of signal when you were not playing any audio prior to going to the balanced connection?
How long was the cable run using the unbalanced line connection?