Instead of consumer electronics I think this question would go best in Electrical Engineering or something.
You could split each of the each of the RCA cables with splitting adapters, and using the extra outputs, make a low-pass filter: http://en.wikipedia.org/wiki/Low-pass_filter
You probably want to set the high-cut limit at somewhere between 80 to 160Hz.
Or instead of making one, you might be able to find a passive low-pass filter circuit kit at a local electronic parts store.
Also, your subwoofer might have a built-in high-cut filter (low-pass filter). In that case you wouldn't need to build any circuits - just split the cable at the source with an adapter and plug it to a subwoofer. However, the subwoofer usually only takes a mono signal. That means if you use a stereo cable, only one side's bass will come out of the subwoofer.
So once you split it in each RCA, use a stereo-to-mono cable if you can find one at your local Radio Shack or something, then plug that into your subwoofer.
And even if your subwoofer doesn't have a high-cut filter, not much of high sounds will come out of a subwoofer anyway, I don't think.
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