Hello! I just got an RH450 amp head, which I have running through a GK 212 neo cab.... sounds GREAT, with lot of thunder! The question I have is.... on the LINE IN, I cannot get the sound to come through the cab, only the headphones. with my other amps, I am able to ply the music from my computer or iphone through the speakers to play along with. How can I get the music to practice with through the cab speakers????? Any help would be GREATLY appreciated! Rock on!