Why would you want a flat response setting on your amp for a non-flat response instrument?
A good sounding amp (be if for bass, guitar, PA or hifi) is not ever going to be on flat settings in actual use.
There would be only one real world application for it and that is calibration.
If your ears won't tell you what a fairly neutral setting is, but they'll tell you what sounds good to you... That's the best we can wish for.