Is the factory cord actually grounded i.e. a three pin connector ? If it isn't, you might have the polarity reversed on the plug or the outlet. This can introduce a hum into the amp which is then fed through the speakers. Sean
Definately call B&K they are more than happy to advise you on potential problems or even suggest a fix. They are only in NY and a pleasure to deal with. A hum that is isolated to the component can be two things... faulty ground or
bad or noisy transformer or power supply...
take the piece over a friends and plug it in... see what happens and if hum still persists.. call B&K.