The diagram doesn’t really depict the purpose of the 2 XLR/TRS inputs. The gain setting of these inputs allowed you to reduce the input signal to 0dB FS, but I’m afraid the idea is to use the interface at the 0dB level shown on its display, which seems to be at about -10dB FS. (Yellow leds = warning mode @ -15dB FS).
View attachment 440652
In my experience, the level of (uncompremised) headroom is what often sets apart budget pro equipment from high quality equipment. Not making excuses for this Behringer interface, I know there are alternatives in the same price range that do deliver. A studio monitoring controller should be capable of doing 4V RMS at minimum.