That would work but it would also turn down the volume when switching between activities which probably isn't necessary.

The best solution would be to have a separate activity that only includes the A/V receiver and put the volume down commands at the beginning or end of the activity. When you select that activity it will turn off any devices that are on except the A/V receiver and since you're using the harmony's Smart State activity switching it will know not to send power toggle commands to devices that are already off.

To use it, select the activity Rcvr Vol Down first, then hit the OFF button.