Your stereo is no where near as loud as a rock concert. Don't sweat it. That said line in near 100 is about what you should need at a medium loud show. Not a problem, line in you don't really want to adjust down (attenuate) too far if you can avoid it.
Also, the high/low/ mid gain setting does not affect line in, just mic in.
And edit to add, ere are 2 reasons to prefer line in, not sure why you would in your case. You mit need to run line in if the signal is too hot for the mic in even on low sensitivity (eg, need to go below about 70 on the input level even when on low sensitivity). Other is if you have a preamp to provide quieter gain. Beyond that, nothing gained by avoiding mic in.