A subwoofer may have one or two input channels to accomodate different system requirements. This answer assumes you have a powered subwoofer, although the same basic princples apply if yours is not. If your subwoofer has a single input, line level or speaker level, and you want to use it with a 2-channel source, your options are limited. You cannot use the speaker-level input, because the two output amplifiers (left and right channels) should never be directly connected to one another. Never. This can damge the amplifier(s). A passive summing circuit could be constructed, but would adversely affect the system's sound quality. If the subwoofer has a single line-level input and your amplifier provides preamplifier outputs, you can sum the left and right channels electronically. This must be done to prevent crosstalk (i.e., provide isolation) between the two channels. Most of these circuits are home-made, which requires some expertise in electronics. You can also use a good quality 2-channel audio mixer, mono or stereo (use one output channel and pan both inputs hard over to that output). A mixer will provide the required input isolation. The end result depends on the quality of the components used in the circuits. If your subwoofer provides two input channels, you can simply connect the left speaker or preamplifier output to the left input and the right signal to its input. Summing is done in the amplifier / filter unit installed in the subwoofer.