I have the same issue.
I love this app! I managed to connect my Samsung Galaxy S6lite to my old Android phone (Honor BLN-L21) and was able to play music from the SD card of the Samsung tablet. However, this does not work with Spotify (premium). I can use the Soundseeder as output in Spotify (using the HiFy app as prescribed), but there is no sound :'(
Android version 11, Spotify version 18.104.22.168
If anyone knows a fix, I would love to hear it.