I don't think it matters too much. I use an Anthem MRX-520 5.1 receiver as a two channel device connecting to it an Oppo 203 and 95. I let the Anthem down mix from HDMI. For analog to my headphone amp, I must have the Oppo down mix and go into those settings to set that.
For your setup with HDMI and optical, let the receiver do it. Probably the simplest method. It knows how many speakers are hooked up and does it automatically without having to go into settings.
Supercharged Song Towers, Oppo BDP-203, Anthem MRX-520, LG B6 OLED
Headphone Rig: Oppo BDP-95, Burson Audio Soloist MKII Headphone amp/ Sennheiser HD800S, Sennheiser HD650