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.
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.