You can also use a splitter on one of your preamp IC's to the amp. After the split one runs to the amp input on your powered sub and the other to the power amp for the main speakers. You can split both lines this way if you desire.
Hooking Up A Subwoofer
How does one integrate a sub in to a system that has no inputs for one? I have an Aragon 8008bb amp and a musical Fidelity A3cr pre amp. This might be of interest to me if I go for smaller monitor speakers. Thanks.