Comment 17 for bug 970965

OK, I am starting to understand.
A solution depends on how the audio signal routing and the midi signal works inside the VMS2.

Here some possible solutions:

1) User Mixxx's mixing engine (in 1.12 it is a 24 bit bit perfect engine, so not that bad :-))
* Configure VMS2 left channel for Master and the right channel for headphone.
* Disable VMS2's Mixing engine:
 a) * Setup the VMS2 in a way that the sliders are decoupled from VMS2 Mixing engine and only emitting MIDI signals.
 b) * Move VMS2 cross fader left and set left volume to max
     * Permanent EnablePFL for the right channel on the VMS2

2) Use VMS2's mixing engine (Situation 2) + Mixxx cueing
* Remove the midi Mappings for Volume sliders and cross faders
* Set Volume slider in Mixxx to max and cross fader to center
* Use the PC Sound card for headphones.
This way you can still individual cue decks and samplers and use preview deck.

3) Use VMS2's mixing engine (Situation 2) + VMS2 cueing
* Remove the midi Mappings for Volume sliders, cross faders and PFL buttons.
* Set Volume slider in Mixxx to max and cross fader to center
* Configure PFL buttons on the VMS2 to listen to the left or right channel. like you have probably done
in the two deck setup (Situation 2)

I am really curious to read if some solution are suitable and why the others are not.