How are you powering the mics? Don't they need phantom power?
You know... if it was just an acoustic guitar, no vocals, I would close-mic it, like I was in a studio. Look into that online... This is just personal preference, but I think your quality would be much better if you close mic'd it then if you were going recording it after it went through the PA/AMP (if that's what's happening). Acoustic guitars close-mic'd properly sound wonderful!
Wether you close mic it or not, here's something I run into with quiet shows often: They are quiet, so I set my levels pretty high to record it loud enough, then they play something that gets louder than my levels can handle, and peak it out. If you can, and you know the person, I would, before the show, set your levels to his/her loudest tune, if he/she will do that level check for you. If this is not possible, be sure to leave yourself headroom for louder parts.
ANd I always have to normalize my tapes from quiet shows... prolly cause I run the levels with a little headroom. My preference is to have to normalize than to peak it out.
Hope this helps dude.