Majik,
Yes, the dsl connection could trigger a ground loop, so try disconnecting it to see if the hum goes away. If a sub is connected to AC power, but there is no subwoofer cable connected to your Denon and there is no hum, then there is no problem with the subwoofer.
I'm still fairly certain the hum can be traced to your PVR/DSL connections so remove those and see if the hum goes away.
Normally, the AV receiver is never the root cause of a ground loop. The loop results from other components connected to the AV receiver. And the loop can occur even when all the components are powered off, except for the subwoofer.
It takes patience and persistence to track down the source of a ground loop but it can always be corrected.
Alan
