Rob,
What a problem!

First, I am not familiar with a 789-50 and it is not listed at Xantech.com. Just what is it?

Second, if I get this right, you have the “wake-up” problem when you use the rear input, right? And only then? The joke answer is to create a macro on your remote control that tells it, say NINE times, to turn on! The real answer: Just use an emitter on the front, BUT:

Are you saying that the Marantz will only respond to the commands from the Xantech volume control sensors if those commands go into the back of the machine? If so, then you are screwed. Otherwise, take the control signal from the family room system and the control signal from the Xantech volume controls and combine them. Xantech has a two-(IR) zone controller that will do this and give you a common out, or you can do what I do: just connect the buses together! Most Xantech products are wired so that multiple bus signals can just be wired in parallel (I have had to insert a resistor with the CC12 and CC62).