bass humming

so my boys got a 07 bmw 3 series with a 750x1 and a l7 12 he is using a prepherial hi-lo converter. now everytime he turns off his radio the sub humms. now i belive he just has to turn the gain on the hi-lo down but i wanted to get some other opions/advice 

sounds like you have a bad ground on your stuff, the humming can be caused by engine noise

The problem is that your amp is not turning off with the radio.  You turn the radio off and the amp stays on, you connected the line out converter to your rear speakers.  The rear speakers move slightly and this produces voltage at the line out converter and that tells the sub to move this in turn moves the rear speakers more and the cycle starts all over again.