You can use Y cables and send one pair of RCA's to the amp and another pair to the sub. Or you can loop through the sub's crossover if it has one. I prefer the Y cable method.
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Put a pair of Y cables on the front left and right outputs of your receiver. send one pair of interconnects to your amp and another to the sub's input. A two channel pre works the same way. Some have more than one output making the use of Y cables un-neccesary. You can adjust the sub with it's own controls. If you want to use the sub's crossover, you will have to go from the pre out on the receiver to the sub's inputs and then go from the sub's outputs to your amp. You won't need Y cables this way.