No need for anything extra at all except a mic or two..
All you need to do is set up one (or two) mics on the side(s) of the stage, facing out towards the crowd. Doesn't really matter what kind, although I'm sure you've got an extra 57 or 58 lying around after the stage is all set up. Run it (them) through the snake and make sure the engineer doesn't run those channels through the house, but does send them to the tape out.
Voila. No need for a pre or any of that stuff. From your original post it doesn't appear that you want a stand alone audience recording, you just want more room sound in your board mix, so that's the way to do it.
Good luck!
Dirk