I assume you're asking about the button located in the lower right hand corner of the 'FireBox Mixer' labeled 'Bypass'.
But I'm not sure what your questioning or why.
That button is positioned where it is because it bypasses the 'Main', '3/4', '5/6' and 'Spdif' options and sends the output of the mixer straight to the FireBox without passing through another gain stage. If you have the line input 12dB boost checked in the hardware setup applet that's where your level boost is coming from.
And you can mix any way you like, bypass on or off. Decide which one sounds better and use that one.
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