This test was with Guns n' Roses Knockin' on Heaven's Door at 44.1 kHz. Setting the Mac Pro optical out to upsample to 96 kHz sounds similar to the iPhone at 192 kHz but has a fuller sound than the iPhone and is significantly louder (even though both are digital signals, iPhone is using USB input of RME ADI-2 DAC FS). Sounds pretty good but I prefer the optical output from my Mac Pro 2012 manually set to the correct sample rate. Forced upsampling isn't the worst solution. Switching back Amazon Music HD app, sample rate jumps back up to 192 kHz, even when playing back CD quality 44.1 kHz sample rate music. Switching back to another music app, sample rate switches back to 44.1 kHz. The app just sets the sample rate to 192 kHz regardless of the original sample rate. I've just tested the Amazon Music HD app on iOS again with an RME ADI-2 DAC FS. Has anyone checked if HEOS does play Amazon HD and switch sample rate? My guess is that HEOS does not yet support Amazon HD Music. Anyway there are different ways to get the same or similar result but of course if you're happy to run a server at home then you have great flexibility & advantages over relying entirely on what is offered via apps and web access. Some services I need to make available to people with only username and password authentication and who I can't reasonably expect to configure a vpn or proxy client so I have to be flexible, but if it was just for me I can see myself just opening a port for wireguard and maybe another for shadowsocks (I'd prefer only wireguard but some mobile ISPs throttle or even drop UDP packets and so sometimes I need an alternative). For different reasons I do run wireguard VPN and sometimes shadowsocks (socks proxy) and also multiple other services on different devices at home (Calibre book server, tvheadend server, mosh, ssh, ssl, letsencrypt). That way you can run a firewall with minimal exposed ports and no services accessible unless you have a secure key.

Another way to do it is to also run a vpn server or socks proxy on your home network and access your LAN from elsewhere only via those.

Amazon hd music