Whatever you choose, I'd make sure they put a mic in front of your bass. Otherwise, they'll probably take a DI from your pick up

. Last festival gig I did, I just used my Solstice blender. Sent the pick up to the monitor and the mic to the house. That worked well. An amp is just an additional monitor if you think about it, unless it's an integral part of your sound (which would be highly unusual as a DB player). In a big festival gig, you're the only one who is going to hear your amp (unless they mic the amp, which is unlikely).
If all the audience hears is a pickup sent to the PA, you'd be better off playing slab. Use a mic for the house, and whatever amp/monitor they have for your own ears.
