I´ve just bought the Audio Interface Fast Track Ultra and use it in combination with the Traktor Pro software. Unfortunately i can not monitor (using headphones). I tried all combinations in the traktor settings, however I can just listen both, the master channel (speakers) and the headphones.
In the fast track ultra settings I can also see some output peaks in the first two channels, but not in the rest.
The A/B button selects the output source sent to the front-panel headphone output. In the A (out) position, the main line output signal (Outputs 1 and 2) is sent to the headphone outputs. In the B (in) position, the S/PDIF output signal (Outputs 3 and 4) is sent to the headphone outputs.
The headphone output level is controlled by the Level control knob. This output level is independent of the main output level.
so if the button is pressed your routing configuration should work. that is if you have an active cue, as well as an adequate monitor level in traktor and the soundcard.