Good afternoon,
Apologies, I think I understand you now.
It sounds as though you are operating the
18i8 correctly. Essentially when the input is set to Mic, Line or Instrument, this is just referring to the level of signal the input is expecting to see. As such, it's quite normal to need to increase the gain on the combo inputs (when set to line) in order to match the fixed gain on the rear line inputs.
As you're no doubt aware, you're simply looking for any clipping on the Scarlett or within your DAW, if you avoid this, you should not have any issues at all.
Hope this helps, please let me know if you have any other questions.
Simon // Focusrite Technical Support