Basically you would like to set the input sensitivity so that the output voltage of the source device is able to push the speaker up to its limits.
The input sensitivity setting in Genelec speakers is defined as the level of the input signal required for 100 dB SPL at 1 meter distance. So if you set the sensitivity setting to the maximum i.e. -6 dBu, it means that the speaker outputs 100 dB SPL at 1 meter distance with -6 dBu input signal level. In another words, when the maximum output of the speaker is 110 dB or more, you will need at least +4 dBu signal to push the speaker up to its limits. With + 6dBu sensitivity setting (min) a level of +16 dBu would be required. If your source device isn't capable of outputting such signal levels, you should increase the sensitivity setting on the speaker.
Usually it's best to set the sensitivity to the max.
Please also see: How does the input sensitivity work on Genelec speakers?