Straight from the Amazon website.
What audio quality does Amazon Music HD support?
Amazon Music HD offers lossless audio in two quality ranges: HD and Ultra HD.
HD tracks are 16-bit audio, with a minimum sample rate of 44.1 kHz
(16/44.1 is also referred to as CD-quality), and an average bitrate of
850 kbps. Ultra HD tracks have a bit depth of 24 bits, with sample rates
ranging from 44.1 kHz up to 192 kHz, and an average bitrate of 3730
kbps.
In comparison, most standard streaming services currently offer
Standard Definition (SD) with a bitrate up to 320 kbps. These audio
files use lossy compression, where details of the original audio are
removed in order to reduce the file size. By contrast, Amazon Music HD
preserves the original recording information to deliver the highest
quality sound available, more than 2x the bitrate in HD and more than
10x the bitrate at the highest Ultra HD bitrate. Amazon Music HD will
always play the highest quality content available, based on network,
device capability and your selected settings.