What you probably are experiancing is a "ground Loop" Ground loops can occur when 2 or more electronic devices, with seperate power supplies are connected together. Simply put, a portion of the AC line signal is transmitted through the ground connections of the instruments, causing the hum. Ground loops are so common, and such a problem, they are one of the reasons thaat professional recording equipment uses 3 wire "balanced" connectors. They are also the reason many keyboard amps have ground switches, which let you change aspects of the ground connection. Hardware ground loop eliminators are available, and do a good job. You can find them at professional audio suppliers. A good UPS (uninteruptible power supply) can also eliminate ground loops, connecting your computer to one will isolate it from the incoming mains power, thus breaking the loop. The fact that your DJX doesn't suffer from the ground loop is just do to the fact that in some small way, its circuitry is different, so in your particular setup, it doesn't create a ground loop. Such is life, and the nightmares a sound engineer's life are made of...
Dave Waldman