Amazon HD Ultra, to DAC via Chromecast Audio - Only seeing 16/44

That is a document listing the supported incoming media. Not what it outputs. You can send 24/96 to a Chromecast Ultra, but you will get 24/48 out of it via HDMI.

Firestick only does 48/24. Also if your amp has Bluesound module that supports Amazon HD natively.

You cant get Amazon HD on a ChromeCast you can only currently cast lossy files it won’t be streaming the hd or ultra HD stream. There are only two effective ways to getit properly use an Amazon link or Bluesound device as they will switch rates accordingly and won’t resample. Any other device cannot currently output bit perfect unless the output rate matches exactly the sample rate of the file, if it doesn’t it will resample to match the output rate of the device, no Amazon apps autoswitch sample rate based on the media, this includes pc, Mac, Android, iOS and any fire devices. It’s a pile of streaming crap, if you don’t care about your audio being messed with then play along with it otherwise I would advice leave it alone like the plague.

Well, it seems Amazon don’t want you to do that.
But - I found a work around which seems to work.
I use my Galaxy s8, and share the screen (using Samsung’s ‘smart view’ to my Google Chromecast TV) with its hdmi into my (old) Panasonic TV. The spdif from the TV goes into my Arcam DAC to my hi-fi, and the Amazon music HD app confirmed it was 24bit (unlike the vague denial when using the Chromecast audio). My DAC does not display bitrate, so can’t say for certain BUT it did sound fantastic!
(the ‘share screen’ from a Moto6 phone did not appear to share the audio in the way the Galaxy S8 did, so may not work on different phones.)
Kinda gave me a buzz to get round Amazon’s pathetic blocking of use on the Chromecast audio device)