This applies to the TwinTrak Platinum
If the digital insert button is pressed in on the TwinTrak Pro then the signal from the DAC is routed to the channel line inputs. If the digital insert button is pressed out then the DAC signal is routed to the inners of the monitor in jacks. If you have jacks plugged into the monitor ins then this routing is broken so remove anything you have connected to the monitor in sockets if you wish to monitor the DAC signal.
If you still cannot hear the DAC signal then ensure the latency-free monitoring section controls are set correctly. Make sure the 'headphones mix' knob is set to 12 o'clock to get an even balance between the channel and DAC inputs and then adjust from there as necessary to get a good mix.