It really depends on your set up. I run two rigs, sometimes both for certain shows:
CSB > spsb-1 > JB3 (20gb)
CMC-8 > spsb-3 > JB3 (40gb)
For the CSB rig, I go line-in +0dB and I will never have to worry about clipping.
For the CMC-8 rig, I also go line-in, usually +0dB, if it is insanely loud, i might go down to -2 or -4dB, just to be safe, but I have never had a problem with those settings yet. There are no high or low settings on the JB3. Also, search around on here, there is no way yo ever want to go Mic-In, i did that once, and it was awful.