I hope this is in the correct section. My current amp which I can't really afford to replace has only two outputs, L and R. I am fairly clueless when it comes to amps (I've designed and built a fair few speakers, but never even looked into this field.) What is the best method (in idiot terms, please) of attaching a sub to my current system? I'd be building it myself, probably a 80-100W RMS. Thanks.