Pertinent information from Apple Insider:
https://appleinsider.com/articles/19/09/17/heres-what-you-need-to-know-about-lossless-amazon-music-unlimited-hd
"HD tracks are 16-bit audio with a sample rate of 44.1kHz, or CD-quality, and an average bit rate of 850 kilobits per second. Ultra HD tracks 24-bit audio with a sample rate of up to 192kHz, and an average bit rate of 3730 kilobits per second. As Amazon points out, most streaming services only offer standard definition quality music, which reduces some of the detail in order to save on file size.
Because the files are lossless, it should be noted that they are significantly larger. A standard song comes in at just around 9 megabytes, where as an HD song comes in at 50 megabytes. An Ultra HD song comes in at 153 megabytes, meaning that devices will be able to hold less music at higher qualities.
This also means that streaming HD and Ultra HD songs will use up more of your data plan, so it's advised that people with lower data caps listen to music via Wi-Fi or download the music for offline listening later.
Additionally, the quality of the music is going to be heavily dependent on how you're listening to it as well. Playing the music through low-quality speakers is going to render a low-quality result."
https://appleinsider.com/articles/19/09/17/heres-what-you-need-to-know-about-lossless-amazon-music-unlimited-hd