Strange that this is happening, but thankfully, the fix is very simple. I imagine that you have 2 1/4" cables connecting the AudioBox USB to the HP4 (one going from Main Out L on the AudioBox into Input L on the HP4; one going from Main Out R on the AudioBox into Input R on the HP4.) Simply swap the cables on your HP4 - plug Main Out L from the AudioBox into Input R and vice versa. That should reverse the channels in your headphones.