Official comment
AFAIK, the headphone & master knobs on the SX are hardware and not midi. I saw with my SX that on screen the master is set to about 70% out and greyed out. This setting makes sure that there is no clipping even with the hardware master fully open. With Virtual DJ for example, when the master output is at 100%, I can't open the hardware button past 50% without getting the red clipping lights turning on. TL;DR: This is probably by design.