Hi,
I use my apple tv in two different modes:
1. Connected to my AV receiver using an HDMI lead. TV is connected to receiver as an Hdmi output device ..Used to watch movies, view photo slide shows, you tube videos and tv shows streamed from ITunes on pc upstairs (technically a Linux server running a windows xp virtualbox machine)
2. Using an optical lead connected from apple tv to AV receiver. In this case the apple TV acts as an airplay device with the TV off (similar to an apple airport express device) . This allows me to stream music from iTunes on pc as well as AirPlay on iPhone/iPad to play music through the AV receiver's speakers. In my case as the apple tv is always on, I just select the optical input on the receiver, hit play on the pc/iPad/iPhone and select apple tv as the airplay device
In mode 2 there is no need to have the tv on. In fact using the harmony one remote the only devices configured for the "AirPlay" activity are the apple tv (just to wake up the device) and the receiver.
Just tried it now and it works with tv off!
You might want to check that the optical input you use isn't assigned to the same input device as the hdmi lead, as certainly on my Sony amp I can combine audio from the optical input and video from any HDMI input to a new input selection
Just to emphasise, from the apple tv the optical lead and the HDMI lead should go to two separate ( unlinked) inputs on the receiver allowing you to use each one independently.
HTH
Dee