I know this sounds silly but,is the sub powered up?Also,knowing what sub you are using will help others help you.Your manual doesn't explain connections?Good luck.
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The model number will help a lot, but, generally, subs sum the inputs they are given, so you only need one cable going into your sub. You can use an rca splitter from the pre-amp into a single cable in the sub, usually the white input. you could also use two rca cables, but that is a waste of cable since the sub will sum the left and right inputs anyway.
Also, make sure the sub is on. For example, sometime the active/passive switch is flipped to active to sense a signal, but the master on off switch is off. I make that comment because some sound should be coming from the sub, even if the connections are off.