The Firestick is a the source from Amazon. (EDIT)...
Directly from Amazon's website, it does not support ULTRA HD audio (24-bit) just HD audio (16-bit).
"Alexa-enabled Echo devices (2nd generation and later), Fire TVs and Fire Tablets all support HD quality audio. The Echo Studio, Echo Link, and Echo Amp support Ultra HD quality audio."
So, you need a device which can receive and decode their Ultra HD audio tracks.
Look at this website for some additional information...
Online shopping for Amazon Music Unlimited FAQs from a great selection at Digital Music Store.
www.amazon.com
What will you be playing it back on? Do you have a good audio system which will benefit from the increase sound quality?