Liz,

This is an example of a pre-made sub cable I have used with success http://www.cablestogo.com/product.asp?cat%5Fid=2018&sku=29118 . I don't see any specs on the web page but it will work well.
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AvFan
Outlaw 976 | ATI 2005 | M&K 850s Left, Center & Right, Surround-55s, MX-70B Sub | Harmony 950 remote | Panamax 5100 | OPPO UDP-203 | LG OLED55B7