Hi Rob,
an Echo can see a Brennan as a Bluetooth speaker and a Brennan can see an Echo as a Bluetooth speaker but the Brennan only allows other devices to access the HDD as NAS or a DLNA server.
There are Alex Skills which can access a B2 as a DLNA server (Our Jukebox) which requires DLNA to be enabled on the B2 and the creation of an 'Our Jukebox' account on the web.
'Our Jukebox' creates its own index of the Music on the B2 HDD as which it stores in the cloud under your 'Our Jukebox' account
'Our Jukebox' can be controlled by voice commends to Alexa but when I tried it I found it a bit kludgy.
John