^ all good advice.
Are you certain that this is isn't ambient noise in the room like an HVAC system. While it may get buried into the noise floor at a louder show you will definitely hear it at a quiet show. As you increase the gain you will increase that noise as well.
This is M10 selfnoise. I'm pretty sure. It's there before and after the shows as well.
When you say the noise is the "before and after the shows", are you saying before and after the set while still at the venue, or did you try everything with the same settings at home, in a very quiet room, mics under pillows / blankets / etc? My first thought was PA or other environmental noise as well. The mic self-noise as well as the M10 self-noise shouldn't be an issue in my experience, unless you're cranking the M10 really high or something is set wrong.
Yes the MIC in is more than safe to use on the M10 with quieter material. Just make sure your MIC SENS switch on the bottom is set to LOW.
Why not post a sample file here for some of us to listen to / look at? That might help narrow down the source of your noise.
You might also want to post your question in the main M10 discussion thread:
http://taperssection.com/index.php?topic=170179.240