The FX Chain input/output sliders are the same controls as the input/output gain knobs. Most of the time I would recommend leaving both at 0 and adjusting the levels of your other devices to send a nominal signal level to the H9000. If your input signal is too hot or too quiet, you can adjust the line level inputs from -10/+4. If this isn’t enough, then I would boost/cut the input gains of the FX Chain. If your FX Chain processing is adding a lot of volume, you can cut the output gain.
When using Kill Dry, the mix parameter becomes your wet/FX volume level. If you are using parallel processing, use the mix parameter to determine the overall volume of the H9000’s FX Chain (after proper gain staging).