In addition to the previous response I would suggest running the bass slightly different than the guitars, both acoustic and electric. If possible try it this way: 1/4" patch cord from bass to a di(direct box), 1/4" short(3 ft or less) patch cord out to a small bass amp, 3 prong low impedence cord(a mic cord)out to the PA channel and mix from there. I've found that bass frequencies are hard to hear on stage in an acoustic situation unless there's a separate monitor amp of some sort. This gives the bass player 'stage volume' control and will help fill out the sound in front of the stage area. And being outside is at best a minor monitorial nightmare. You're going to need all the help you can get. Have a good gig!