If you are using an MD to record LOUD concerts, use line in and a battery box. The pre-amp will just raise your volume, which you do not want. If the mics cannot feed enough volume at line-in, use mic-in on the low setting. If that does not work, use the high setting. If that does not work, try the pre-amp.
So, how can you ever do that at the concert? You can't. Put on some loud music at home and try it. It will help you know ahead of time what the setting should be and give you practice in setting up. I hope you enjoy what you are doing. This is fun. And with practice you can create some first class recordings.
Cheers