1- I'm using the monitor out" to send program material back to the TV. Video works fine- but audio volume is barely present & unusable. (Note the audio signal to amp is fine).
I'm not sure quite how the monitor out left/right channels work on the 970, but I would expect them to be affected by the processor's volume control - at which point, you would have two volume controls in series, which would likely present itself as you describe.
3- The 12 volt trigger to the 750 no longer works.
Just to be sure - is the trigger cable going to the trigger output and not to one of the IR ports?