Took a bit of fiddling, but I think I’m there: it appears librespot is not too happy about ALSA opening USB hardware directly. A workaround is to use ‘plughw’ instead of ‘hw’.
Find your correct ‘card’ by issuing ‘aplay -L’ (capital L this time). On my Cubox, the one I’m looking for is listed last (Chord Mojo USB DAC; yours will differ):
plughw:CARD=Mojo,DEV=0
Mojo, USB Audio
Hardware device with all software conversions
In the service definition file, change ‘–device hw:1’ to ‘–device plughw:CARD=Mojo’ (change Mojo to whatever is listed through ‘aplay -L’ for your DAC. You can leave out the ‘,DEV=0’.
Then, issue:
systemctl daemon-reload
systemctl restart spotify-connect.service
Let me know how it goes!