Apple Music is a mess. I use it, but it isn't fun to set up. I DO NOT use my Apple TV 4K for music except when wanting to hear Dolby Atmos tracks from Apple Music, BPO or Tidal. The Apple TV 4K will output everything at 24/48 by default. So the lossless 16/44.1 tracks get output at 24/48 over HDMI. It does not support the hi-res tracks. No, I'm not promoting hi-res as much as bit for bit output. Conversions for output shouldn't do so but can result in audio output issues.
I was also wondering if Bluetooth was being mistaken for Airplay 2. Airplay 2 does support lossless audio but there are kinks. If Dolby Atmos is ON for Apple Music, it will not work over Airplay 2 and a lossy AAC signal will be sent instead of the lossless ALAC signal when Dolby Atmos is turned OFF. It doesn't help matters that Apple Music is not accessible using HEOS, MusicCast or Play-Fi when using any of those apps on different AVRs. Apple Music will send lossless 16/44.1 over Airplay 2 while Amazon Music will actually do it up to 24/48, the limit of Airplay 2.
Apple finally unlocked the HDMI port on my Mac mini M1 so that 24/48 is not the only bit/rate combo available for output. I've used Amazon Music Unlimited, Apple Music, Qobuz and Tidal apps in my Mac and all can output 16/44.1 - 24/192. I can't honestly tell you if one sounds better than another as they all sound great. Apple Music does not support Exclusive mode and actually killed it for Amazon Music in MacOS Ventura. Tidal Exclusive mode still works and bit/rate will change for lossless to hi-res tracks and even silly MQA unfold works using Mac. There is a third party app for Apple Music lossless switching. Using Airplay from my iOS devices and my Mac to my receivers using any of those services also sounds great.
With your iPhone and Marantz on the same network, you will do fine to use Airplay 2. You should make sure of a few settings in your iPhone for Apple Music. First, go to Settings and scroll down to Apple Music:
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Then, Tap Music for its settings:
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Notice Dolby Atmos is OFF here. It doesn’t work over Airplay 2 so just turn it OFF or any Dolby Atmos track will be sent out in AAC and the Lossless indicator will not be present. Then, go to Audio Quality and turn Lossless Audio ON:
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I just leave Hi-Res Lossless ON for everything but Lossless up to 24/48 can also be selected.
Now, when streaming from the iPhone using Airplay 2, lossless audio will be sent.
Here is a pic of a track that supports Dolby Atmos with Dolby Atmos ON. It is not lossless and will not go over in Dolby Atmos using Airplay 2
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After turning Dolby Atmos OFF, the Lossless version is available and sent over Airplay 2:
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You can turn Airplay 2 ON from the Control Center or from within the Apple Music app track. The Airplay icon is at the bottom of the screen when playing a selected track:
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Tapping the Airplay icon will show the iPhone as the current output device and also show any Airplay supporting devices on the network to which the music can be sent. Your network would show at least your Apple TV 4K. When sending the music to a receiver on your network via Airplay 2, the volume buttons on the iPhone will control the volume level of the receiver:
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The Apple TV 4K has similar settings for Apple Music, though, it does not support Hi-res and so Lossless up to 24/48 is selectable and Hi-res 24/192 is not an option at all. Dolby Atmos can be turned OFF as well in the Apple TV 4K Apple Music app so that only Lossless two channel versions of tracks are played when Lossless is ON.
If just using Apple Music from iPhone, set streaming and downloads to Lossless. They will both be ALAC(Apple Lossless Codec.)
Interestingly, some Dolby tracks in Apple Music are 5.1 and labeled Dolby Audio while some are actually Dolby Atmos. Turning Dolby Atmos OFF means Dolby Audio tracks are also played back in lossless two channel versions when Lossless is ON and Dolby Atmos is OFF.
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The Dolby label will be present when using iPhone speakers with Dolby Atmos ON but will not be present when using Airplay if Dolby Atmos is ON.