In your power up macro add a variable (click on the v=1 in the macro box): VAR(Mediabox = true)

Then in all of your power off and power on macro's for your other sources put the following:

If (variable(mediabox,true)
mediabox power command
Var(mediabox = false)
else
Var(mediabox = false)

You have to always remember to set your variable to false.

You could also add the same if then statement as above in reverse to power it on so it checks the variable first to be false before issuing the power command.