I think you also have a line-level out on the sub. Just chain them together. Being that a subwoofer is non-directional it you don't really need a left and right channel sub. You could put one sub up front and one behind, etc. Also, your line out on your receiver may be attenuated? Is that a possibility? Not sure. Also you may be having phasing issues. Try switching the phase on one sub, then the other.

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