I agree that Amazon Music HD seems the most interesting option, and more if you are prime, but the application leaves much to be desired. Also, the lack of "Amazon Music HD Connect" is, in my case, inexcusable. To top it all, the compatibility with HEOS is pitiful.
I am an iPhone user (although wanting to go back to Android), but I am not considering Apple Music because I am not going to use iTunes. In addition, it would make my subscription more expensive, since I share a family account with friends, and at Apple it is complicated by the restrictions they impose.
I have been a user of Spotify since it was a Beta and as an application or software it is undoubtedly the best, but it is taking too long to join the Hi-Fi. Until recently the 320 kbps were insufficient but "salvageable", because the quality of my devices did not give much more, but for a couple of weeks I have a Denon AVC X3700H + Dali Oberon 5, so the 320 kbps of Spotify are laughable.
I've been using Tidal ever since. The application is not bad at all, it has Tidal Connect which, although it is not compatible with my new Denon, the integration with HEOS is quite good. The music recommendations that he makes me daily and weekly are improving with the passing of the days and the audio quality with respect to Spotify is abysmal, although I certainly like it less than the quality that Amazon offers, not only in terms of "UHD "but also in CD quality (in fact I have not been able to test the Masters, since my Denon is not compatible).
For now, I will continue with Spotify (although using Tidal), but when the 3 months of promotion I have are over, if Spotify Hi-Fi is not a reality, or its quality is not up to par ... I will migrate.
I take the opportunity to complain about the poor compatibility of Denon ... Neither MQA, nor Tidal Connect, Amazon sucks in HEOS, and HEOS itself is slow and clunky ... Of course, if I got to know this before I would have opted for other brands.