Is the oppo very near your AVR or pre/pro? If so, try moving it far away and see if the hum goes away. I had to replace a CD player near my AVR as it was creating a 120hz hum through my amp. New CD installed, no more hum. If your hum returned when the Oppo was plugged in, that is the first place to look.

Good luck, that noise is irritating!

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire