Headphone Sockets on AV amps

Last night I was watching a film, and SWMBO decided to retire early Our bedroom is next to my study which is where I watch my choice of program. So, I had 3 choices

  1. Switch off and start again the next day

  2. Turn the volume down very low and hope I didn’t keep her awake.

  3. Fiddle with the Oppo BluRay and change it’s output to stereo, and continue listening on headphones connected to the Arcam AVR850 headphone socket.

So, why don’t AV amps automatically downmix multichannel audio to stereo for the headphone socket? Seems to me to be an obvious omission in their planning - unless there’s some obscure setting in the Arcam’s setup which would enable that?