Right....no variables on the mx-900 yet so let's forget about that route for now.

Find and utilize discrete commands for power on and power off for all of your devices if they exist and reference those commands during your source/activity macros so that the desired source and its associated equipment are ON and any others that you wish to be OFF are turned OFF. Variables and harmony style programming which at times will require an ask and answer technique to determine the state of a device are band-aids for poor programming and work-arounds for devices which do not have discrete power commands.

Here's a basic example to get you started:

TV - ON
delay
Amp -ON
delay
Cable - 0
delay
Cable - Exit
delay
DVD - Off
delay
Amp - TV/DBS
delay
Amp - Surround
Goto Cable Page 1

Notice I typed ON and OFF and not power which would assume a toggle command for both on and off? So this example macro would set the system up for watching (via video switching in a receiver) and listening to a cable box (through the receiver) and turning OFF the dvd if it was ON but not ON if it was OFF. Mess around with this type of string in your macros and you'll get it.