Two issues:
1) H9 is designed for guitar level, so a hot line level will probably overload it. Note that people talk about "line level" as if it is a clearly defined thing, but in practise there is a wide range – +4dBu is probably a nominal level – peaks may be much higher.
2) H9's inputs and outputs are unbalanced. You can connect it to a balanced input or output if your connections unbalance the input(output) – this often happens automatically.
At the end of the day, do what makes sense. If it clips, drop the level. If it hums, you are probably not unbalancing it properly. Or you have a dread ground loop.
