The Apple TV 4K sends Atmos out as 7.1 24/48 PCM. It’s up to your receiver to manipulate that into 5.1. For the most part that’s a function of how you have your speaker settings set up.I’m in the same boat - just have a 5.1 system and Oppo 103-D. I purchased an Apple TV HD yesterday and will sign up for Apple Music so I don’t feel left behind reading all the “Listening to now in Dolby Atmos” posts, haha. But I just spoke to Apple yesterday and asked what would happen if I selected a Dolby Atmos song, not having any Atmos compatibility with my receiver, player or TV. They said that Apple TV would down-convert it to Dolby Digital 5.1, so I thought that was good news. At least I don’t have to limit myself to Dolby Audio selections.