Disconnect the cable TV coax and see if the hum goes away. If it does, buy a coax isolation device like the Mondial magic box (expensive) or the similar device from Radio Shack.
Try removing the line conditioner, and any other surge suppressors or similar devices that any of the components may be plugged into. If possible, plug ALL components into the same ac outlet pair (using a power strip which does not include a surge suppressor, if necessary). Or, alternatively, plug them ALL into a single surge suppressor or conditioner, which in turn would plug into a single outlet.
At the very least, avoid having some of the components connected to an ac run that originates from a different circuit breaker than the other components are on.
It's not clear if the 0.4 volt difference in chassis potentials are with or without interconnect cables in place between those components. If with, it is strongly suggestive that the shield impedance of the interconnect cables is too high, and the hum might be reduced by using a better interconnect (or shorter interconnects). Actually, a better and/or shorter interconnect (resulting in a lower shield impedance) might help in either case.
See the following excellent reference, starting with Figure 1: