Answer to "Where is the IR sensor?"I can now control the volume of my Pioneer MCS-333 HTiB. My understanding how Apple TV 4Gen learn my Pioneer (PR) remote is: the box learned from PR; the box will push the program to control the volume of Pioneer to Siri Remote (SR). I thought the Apple TV system will learn the IR sequence from PR thru SR. I had already tried turning off the Apple TV. SR can still pump up the volume or down the volume of my Pioneer.