The Aux input is provided mainly for recording purposes. The Brennan Website says:
Select Aux on the B2 remote control. This will reboot the B2 in a mode
that allows the Raspberry Pi to make recordings - but it disconnects the
hard disk and internet. You should now hear your music source playing
through the B2 speakers. Though nothing is being recorded yet. Now is a
good time to play with your connections and volume levels. You may
notice that if you touch the tip of one of the connectors on the lead
feeding into the B2 that you get a crackle or hum - this is good and
it's often a quick way to check that side of the system is working.
I'm wondering if this different mode for the Raspberry Pi doesn't support Bluetooth Out ?