I have a D-150A Series II amplifier that I want to use to power a single 18" subwoofer. When I plug the signal from the mixer into the amp's input (1/4" speaker cable), a blue flash occurs inside amp and all the other speakers in the PA starts making a terrible booming/clipping noise (keep in mind that I didn't even have the output connected to the sub). I only tried it for about a second, and then realized that something is really wrong. I'm not sure if the amp is bad or if I'm missing something. Is there an easy way to test the amp? I don't understand what could possibly be happening that is causing the other speakers to mess up. Does the blue flash I see inside the amp mean that something is broken? Please advise.