[testphase]

what you will miss is Full True HD Dolby Atmos and DTS HD:X as decoding to PCM discards the meta data.

Got it. So if I don't have height speakers, my listening experience will be the same?    

Yes - for most audio tracks all you are doing is moving the HD Audio->PCM decode process from your AVR to your Fire TV Stick.   There may be some minor differences due to the lack of metadata getting to your AVR (mainly if you use Dynamic Range Compression or similar processing?)

What I don't know is whether the Fire TV stick supports 96kHz and 192kHz PCM (rather than just 48k)  There are a small number of Blu-rays with 96 or 192kHz audio tracks which may be downsampled to 48kHz if the Fire TV Stick doesn't support >48k  

sorry for the late quote, but firetv stick 4k can only do 48khz, i tested it. so if some one have a decent sound equipment will be disappointed. i get a zidoo z9s for local playback instead (libreelec, coreelec all have the subtitle issue, too bright under hdr), all the streaming will be done with the firtv stick.