I just got a used Carvin R600 Series II head, and I don't have the cabinets yet to hook it up to try it out, so I plugged headphones into the headphone jack, plugged my bass in, and tried it with no speaker load. Something is very, very, wrong; all I got was a very weak, horribly distorted signal. I've been over all of the controls dozens of times, front and back, and don't see anything that would cause or change this behavior. Can anyone help me figure out what's going wrong? Was the amp damaged during shipping? Help!