Hi Luke --
I also use my Sonos speakers and system with the B2. What you need to do is control everything via the Sonos app, not the Brennan. You start the playlist on the Brennan web interface, and then you'll see it load on the Sonos queue. You can then randomize it there using the Shuffle icon.
One thing to remember: if you want to have the entire playlist load on Sonos, you have to click on the first song in the list. For example, if it's a 200-song playlist and you click the 71st song on the list, it will only load songs 71-200.
As for playing all the music on the B2, just update your music library on Sonos, wait for it to finish indexing, click Music Library, click Songs and then choose Shuffle. It will load however many songs it brought over from the B2 in random order.
Good luck!
Joe