Definitely follow other's recommendation to add an additional activity. This is how I have it.

Problem with your approach by going into Devices to turn on the receiver is the Harmony will still try can control the volume through the TV. Or you could leave the device mode on and use the LCD screen to control the receiver volume. But that is a pain. That's the beauty of the Harmony, just push another activity and now the volume controls the receiver, the up and down channel control the cable box and the

I have three TV activities. One with TV+Receiver+PVR (what does PRV mean anyway????), one with just TV+PVR (mostly for the kids so the receiver does not wake us up) and one for only TV (for the times when we are recording two shows and want to watch a third. We just use the cable feed directly to the TV.