This was on another Amazon thread, not sure if it will help you.
Amazon HD resides within the Amazon Music service on the Node, its just not very well implemented yet. Hopefully this will change. Unlike Tidal and Quboz where you get album thumbnails that show what the resolution is, Amazon Music just shows albums as a list, with no way to know what the resolution is until you start playing. Once playing, the album resolution will show in the now playing box. I haven’t found a way to search by resolution. This is with the BlueOS app on iPad. Hopefully, Bluesound will issue an update to address this. I’ve signed up for the 90 day trial of Amazon HD but won’t renew unless there is a change in the Bluesound app. If I’m missing something, hopefully someone will point me in the right direction.