If you crank the output of your Fireface too hot you’ll overload the converters, This will result in digital clipping (not good). Purpose is to “gain stage” things such that each part of the audio chain is working at it’s best level.
Adj the mic pre’s on the Fireface so you see no clipping indicator on the FF, but you have a good signal level coming into Cubase (-3 db is about as high as you want to go…-6 is probably better, just dont let anything get up to 0).