Not a stupid question, but a fairly simple answer: the sound should be the same on the headphone and the left/right output jacks. If you crank the Stereo knob with an "Init" patch and play some keys, the panning of the voices should be obvious on both the phones an main outs.
Edit: I'm not familiar with your audio interface, but make sure there's no mono monitoring kind of functionality in effect (a possibility with such a flexible interface).