Does that require Sonos speakers with Alexa to the FireTV? Or are you using a different Sonos product?
My impression with Amazon Music HD Unlimited via Firestick 4K was it sounded weak compared to Apple Music Lossless via AppleTV 4K. Possibly just my own ears but I jumped ship to Apple.
I also felt Spotify and Deezer via my iPhone using Airplay were also both superior to Amazon Music HD unlimited. I have not compared Tidal or Qobuz
I hear that Spotify will have updates soon to a lossless tier, but no Atmos is planned.
I was not a fan of my iPhone being the hub of my music. I definitely like how I can assign that duty to my AppleTV 4K with Apple Music. And the Atmos music on Apple is pretty cool. I know Shield also supports Apple Music Atmos as well.
As far as the Firestick goes for TV, it works great for most apps. My xfinity beta app is running on my Firestick in 2 rooms and the 2019 Fire cube in my theater room. I use that for NFL and NHL mostly. I definitely prefer the AppleTV 4K but it doesn’t support xfinity. I also have issues with both Disney+ and HBO Max apps on Firetv that work perfect on the AppleTV 4K. So I use both streamers in 3 rooms depending on apps.
Interesting how varied one’s experiences can be. Replying to parts of your post;
Sonos now recognizes Amazon Music as a steaming service directly through their app and plays 24 bit hi-res and 360/Atmos files. It sounds great in my living room here I have an Arc/2 ones/sub combo. Love the Atmos affect.
I used Amazon about three years ago and thought the sound quality was very good but the UI sucked so I went to Tidal and then to Qobuz most recently.We have an Apple Music family plan so I experimented with that via ATV 4K and also really enjoyed their Spatial playback although I found those albums difficult to search. I didn’t care for masters or lossless tracks through ATV compared to Qobuz through my Node. To me it wasn’t even close.
Interestingly with the 2021 4K Firestick and Amazon I get a very warm and open sound on two channel hi-res tracks, rivaling that of Qobuz/Node combo. I ordered a 4K Max stick (as it also have the Eero 6 fiber modem and the navigation is suppose to be super fast) which will also play Amazon’s Spatial files. I may literally replace the ATV and Node in my rack with a one-piece solution that takes no shelf space.
Lastly, the app interface on Amazon is better then it used to be, color me impressed and especially at $8/mo.