The Sub Mix outs are directly tied in with the Effects settings. Any signal sent to Send 2, will come out Sub Mix 1, and any signal sent to Send 3 will appear at Sub Mix 2. Keep in mind that even though you're sending these signals to your effects sends, the Sub Mix outs will only play the dry signal. There's no way to apply effects to any sounds coming out of the Sub Mix outs. See the diagram on page 222 of the manual for a drawing of how the routing is implemented.