i have a sony cdx-m8805x the installation booklet tells me that i can plug the s/woofer(mono) out lead into the sub speaker but when i do this i get no sound to the sub. so why have a lead that you can plug stright into a sub that people are telling me i need a amp for ,when the booklet shows me it is going into a sub speaker. i also have audio outs to use with a amp