Only stereo sound from Apple TV over HDMI to multi-channel surround preamp [SOLVED]

I have my Apple TV 4K connected to my multi-channel surround preamp via a direct HDMI connection. No matter what settings I changed on the Apple TV, I would only get stereo sound.

I discovered that if I changed the settings on my preamp for the Apple TV HDMI input so that instead of "auto", I forced the mode to "multi-channel PCM", I started hearing full 5.1 surround sound. This worked for a Dolby 5.1 track, and even a Dolby TrueHD track (played by the VLC app streaming an MKV file).

It looks like the AppleTV doesn't provide the information required for the "auto" mode of my preamp to detect that it needs to switch to multi-channel PCM mode, so only by *FORCING* multi-channel PCM mode for the Apple TV HDMI input is it possible to get anything other than stereo.

In short, I strongly suspect this is a flaw in the Apple TV 4K (because all my other sources work fine with my preamp set to "auto" mode) and the only way to work around it is to force my preamp to expect *ONLY* multi-channel PCM audio on the Apple TV input HDMI connection.

The on-screen documentation explicitly says that the Apple TV 4K converts all audio types to multi-channel PCM, so I think this workaround will enable you to get the best sound for anything you play on the Apple TV.

I'll be interested to hear others' experiences.

-PolarWeasel