Originally Posted by OldNewbie View Post
An RJ13 is typically a remote control for bass boost. If there is something connected you will likely have a controller with a small knob installed somewhere to allow you to increase the bass. It is not a signal input. Do you have a photo you could post?
Check this. Also, how many sets of RCA cables were plugged into the amp? I'm wondering how the sub channel gets signal? If it's only two sets, you might have bumped the input selector switch that reads, "2Ch | 4Ch | 5Ch"