Humming noise when my bass knob is plugged in and turned up. Whats the problem?

I have the Lanzar Maxp1200 amplifier. I recently took it out of my brothers jeep because he was borrowing it and I need it back because a have a new woofer on the way. When I hooked it up in my room and turned the bass knob up the humming got louder. With the bass knob all the way down there is no humming. I unhooked the bass knob and adjusted the gain and bass boost on the amp manually with a screw driver and there was no humming. Theres only humming whenever I have the bass knob plugged into the amp and turn it up. I never had this problem before and neither has my brother the whole time he was borrowing it. If anyone has any kind of idea of what the problem might be I'd appreciate it a lot.