Mark,

in case you haven't resolved your issue with the humming: please take a look at all cables that are connected to GROUND in some way. This could be any of the following types:
- power supply cables
- antenna cables
- connections to LAN/router/telephone

Now disconnect all cables that go either into the receiver/amp or the Oppo except:
- the 2 power supply cables (plug them into the same power block or 2 close wall sockets)
- the cables to the speakers
- the cables from the Oppo to the receiver/amp.

Do you still have the hum? If so then pull ONE power plug, turn it 180 degrees and put it back in (pins going into the other hole this time).

The hum should most likely be gone now.

If so then reconnect all the other parts of your hifi set one by one. Does the hum come back? After reconnecting which cable?

Good luck,
Thomas