First, thanks for Mixxx, it's a fantastic program!
My hardware mixer has a separate crossfading knob, labeled simply "Mix", that controls the relative volume of the two decks in the pre-fade only. This is very useful as it means you can modify the volume of the track you're mixing into without the audience hearing. It seems that the Main/Pre switch does not provide this functionality of balancing the two decks. If you try this using the gain, the audience can hear it, likewise with the faders.
Is there any way to do this in Mixxx?