Often times hum is generated through ground incompatibilities between CATV and other components in your system. Here is an excerpt from our Model 750 Owner's Manual that may help:

To determine if a cable TV/Sat connection is responsible for the hum, first turn all components off. Disconnect the cable TV feed to your system at the first place it connects to your components. Alternatively, disconnect the cable TV wire where it is connected at the wall outlet. Turn your system back on, and listen if the hum has disappeared. If removing the cable TV feed has eliminated the hum, you will need to insert a Ground Loop Isolator.