With a Battery Box, Mic In or Line In?
I hate to give a lawyerly answer, but it depends. My Baby NBox sounds better with Mic In rather than Line In, so the answer isn't clear. I would record a bunch of shows or stand in front of your stereo speakers at a reasonably high volume and do some tests.
The battery polarization is another issue, and without seeing your battery box, the springy thing is usually on the negative terminal (-) and the flat part is usually on the nipple looking terminal (+), but again, I don't know your battery box. Try out different directions. I would think that one would work and one would not.
Bottom Line: Test.