This from the Brennan website:
"The Spotify Connect function can be a little bit hard to find (well it was for me).
Once connected the music will play through the brennan - not the phone/tablet.
Because Librespot is an independent app the Brennan doesn't get to control the Spotify music - you control it from the phone. In fact Librespot overrides (or punches through) whatever the Brennan plays. You can adjust the volume on the Brennan however.
To disconnect you need to select This Phone on Spotify
You can find a remove Librespot option in the Advanced menu if you decide you don't want it or if there is a more up to date version in future."