Offizieller Kommentar
Because the DJM-900NXS2, like most DJMs when used with rekordbox, operates in External Mixer mode, meaning the individual channels of audio are sent to the mixer. Because effecting the master audio would require the signal path to go through the master channel, then back into rekordbox, and back out to the mixer's outputs, that would introduce an incredible amount of latency (plus additional audio I/O), so unfortunately you can't select those channels when using a hardware mixer.
Consider using the on-mixer FX set to the MASTER selector instead.
