Obviously I wanted the convenience of streaming an MP3 to the amp versus using a CD player.
I disagree that all solutions to this problem need the same complexity inherent in the Sonos and that the user experience could not be improved.
Keep things modular: get something like a Raspberry Pi with a USB soundcard and plug that into your good ol' amp.
Using that with shairport-sync to cast to a Woburn speaker gives me more than adequate results and I'm not even using a USB DAC nor digital audio output via HDMI, just the Pi's basic line-out.
If you added the functionality of the Sonos to the Rotel poweramp, it would have the same complexity-based problems.