正式なコメント

@Hristijan > The DDJ-RB has a standard +4dBu line level output for the RCA master output.
As the driver for the DDJ-RB isn't designed to be used in EXTERNAL MIXER mode, that was likely a glitch that you were able to engage it. The only thing that may have made it louder is if you also added "Booth Output" to the master, as it would have been sending additional signal.
My recommendation is to ensure that you're running a master signal that is close to the red, but not pinned there or clipping, and then on whatever device you're connecting the master output to, turn up the level so it's appropriate for the input signal level.