Why doesnt my Bazooka subwoofer work 100% of the time?

I have a brand new Kenwood aftermarket stereo. While my tuner(local radio) is on the subwoofer works 100% of the time. But when the CD player or my Sirius radio is on, the subwoofer only comes on when the volume is above 17 or so. When my volume is above 17 or so my sub sounds great but as soon as you turn the stereo down the sub turns off....I know this isnt how its supposed to work!!! But no matter who I ask everyone has the same freaking answer...."hhhmmmm, thats strange, I dont know"....and I do not want to take it to a pro so they can charge me $65 to just hook up wire...PLEASE HELP!!!