Assigning Computer out put to channels

I just received my new 900 yesterday and love it. I do have one question I have not been able to figure out yet. When I play music from iTunes the signal is sent to USB 1/2 on Channel One. I would like this to be on USB 5/6 on Channel Three. I didn't see where this was an option on the driver for the computer. I have installed the up to date driver and firmware. Any suggestions on how to change this?