Hiss in the headphone circuit is a known “problem”. It is not going to tape.
Setting the mic gain in the menu depends a little on the mic. If the signal seems weak, try changing it to -60 which will make it louder.
The meter goes red at -12db, not 0. So, you have some headroom to play with where that particular meter is concerned. For a good, solid signal you want to see peaks into the red.