Working for me with a pi4 and allo digione HAT (spdif out to an external DAC) 
On a fresh 64 bit install of raspberry pi os lite …
Recommended/original plexamp.service didn’t start on reboot … noticed some errors related to node and groups?
Changed to a user service (~/.config/systemd/user/plexamp.service)
(note, I think this also might help prevent issues related to different user names)
Made some changes to the service file …
[Unit]
Description=Plexamp
After=network.target
[Service]
Type=simple
ExecStart=/usr/bin/node /home/pi/plexamp/js/index.js
Restart=always
[Install]
WantedBy=basic.target
Maybe on a lite install (no desktop environment, for headless-only use), the multi-user target is never reached due to no desktop environment, so then the original plexamp.service didn’t start either ? … switched to basic.target
systemctl --user enable plexamp.service
systemctl --user daemon-reload
reboot, check again …
systemctl --user
now plexamp.service is running
