I have a little device into which you can plug a 1/8" audio cable and it transmits audio via Bluetooth, so that you could use a Bluetooth speaker.
I have a UE Megaboom speaker, which will be louder than your Boom, but maybe not loud enough.
One feature they have in the Boom app, "PartyUp," allows you to connect a bunch of UE boom line speakers and play in sync. I've never had more than one Boom device with which to try it, but if you want to borrow my BT transmitter and Megaboom speaker, you're welcome to them.
- Jay