According to Denon and Marantz, since the update of the HEOS software (and the app updated as of 1/11/24) you can play HD and better quality. But, the info is sort of ambiguous. It doesn't really differentiate from playing via the HEOS app, vs casting via the Amazon app. Although, HEOS is launched automatically (but I don't have to interact with it)If you are casting to your AVR from the Amazon Music app and have enabled the Heos skill (which you’ll need to have done to make your avr visible to the amazon music app and Alexa) then you may be getting just SD - that’s all I got in the past when I had a Heos Link HS2 but their firmware (and Alexa) may have moved on since then, but I’d doubt it. You used to be able to open the Alexa app and see what was playing on your account and it would show the HD badge if that’s what was playing - might be worth seeing if that shows the resolution as for some reason the Amazon Music app doesn’t want to show that.
I ditched my Heos link when the WiiM Mini came out and get all the way up to 24/192 no matter which app I use. I feed that over optical into my amp which does display the playback resolution
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With the addition of Amazon Music HD, you will now be able to play Ultra HD music on supported HEOS systems from the HEOS App when choosing Amazon Music.
Ultra HD means at least 24-bit 48kHz and up to 192kHz, which means it sounds significantly better than most other streaming services.
What is required?
Because the ability to play Amazon Music HD on HEOS products is made possible through an automatic software upgrade (assuming your product is internet-connected), all you will need to stream at Amazon Music Ultra HD resolution is an Amazon Music HD subscription. If you have any of the HEOS products below, your product should already be Amazon Music HD ready, provided your product has been connected to the internet to receive the automatic update."