Was thinking of putting the BB into the “aux in” jack on my guitar amp. Is there a workaround to get it loud enough at all gigs, since the amp’s volume controls don’t affect that jack?
If it isn’t loud enough there isn’t much you can do, you could increase the main output and then reduce the gain for your guitar input etc… Or you will need to plug the BeatBuddy into a socket with a preamp. You could possibly try the headphone output, but it will probably not work well or give you extra noise.
Nowadays, Aux In is designed to be compatible with a headphone output. If you have a reasonably modern amp, it should work. Worked great with my BB and the aux in of my mid-2000’s bass amp, a Ibanez Promethean.
It’s not going to sound great through your guitar amp.
That being said, get a cable that goes from the 1/4" outputs [BB] to a stereo mini [your guitar amp]. I found that is louder than going from mini to mini.
Also make sure that both volume knobs (main one on top as well as headphone volume wheel on side) are turned all the way up. This is a pretty obvious thing to do, but sometimes it is the obvious things we overlook. 
Thanks for this suggestion. I tried it and the bass drum sound can really be heard using the aux-in jack; was almost non-existent when using the guitar-in jack. Seems it could be even louder yet if the BB's output was pre-amped before entering the aux-in jack. (Amp is the solid-state Fender Champion 100-watt, with 2-12" speakers.) Does anyone know the specs of suitable preamp to use?