Connect from the effects SEND on the back of the DJX750 to the inputs on your external effects processor. Then connect from the outputs of the external effects processor to the effects RETURN on the back of the DJX750.
The MONITOR signal is taken at the SEND output and routed to your external effects processor.
The signal at the SEND connector is identical to the headphones signal and is selected with the MONITOR CUE buttons.
The externally processed signal is added to the MASTER output signal via the RETURN connectors.
The effect signal volume may only be adjusted at the output control of the effects device itself.