I have the Asus ROG Impact (ITX motherboard). The Q-Cable that comes with it doesn't have any "+/-" markings, and the manual doesn't state which pins on the motherboard are "+/-" either. I found the answer though. The Asus Z87-Pro ITX's manual does state which pins are "+/-" for the Power LED and HDD LED. The pins are oriented differently, but referencing both manuals tells you which pin position is which. The colored wires on the Q-Cable connect to the "+" pins, the white cables to the "-" pins.
For the speaker connector, the "+" case speaker cable connects to "Pin 1" (the bottom position pin) of the speaker connector pins.