Quite a few people run systems with a Raspberry Pi, a high-quality I2S DAC+amp, and a set of (normal, non-internet-connected, firmware-free) speakers. There's a

Quite a few people run systems with a Raspberry Pi, a high-quality I2S DAC+amp, and a set of (normal, non-internet-connected, firmware-free) speakers. There's a tiny industry of producing hardware and software for this situation. "Usability" means very different things to different people (after all, the reason Sonos asks for location access is to improve usability), but maybe look at https://volumio.org/ https://volumio.org/, http://www.runeaudio.com/ http://www.runeaudio.com/, http://moodeaudio.org/ http://moodeaudio.org/. Or just install MPD and Pianobar for that old school experience.