Fire stick 4k can do 24bit FLAC Comparison Table for Fire TV Devices | Amazon Fire TVI think the bottle neck is the FireTV. Your Denon has the HEOS app just load it on your phone/tablet and you can get full 192/24 from Amazon HD directly streaming to the AVR.
The hardware may support it but not the Amazon HD app as I can't get any better then 16bit on my Shield using the app where HEOS works on my Denon x4500h. Of course you need to pay the $14 a month for the premium Amazon HD service. I tried the free trial but not worth it for me.Fire stick 4k can do 24bit FLAC Comparison Table for Fire TV Devices | Amazon Fire TV