I used to have same issue, went out and got a good wall outlet ground checking device. it turns out my wall ground came loose, and many others around the house. Got it fixed, and the hum went away. Also, a good AC conditioner helps.
Where is the hum coming from? If it is coming from the Pre-pro itself then you may have an issue called DC Offset. It's usually caused or exacerbated by dimmers in the house or a particular appliance or a combo of other factors. The bottom line is you need a unit to take the DC off the line. PS Audio had a unit called the him buster that did just that. They no longer make it last I checked.
Emotiva started selling a unit to deal with DC Offset. You will find it here;
If the hum is coming from the unit and specifically the transformer area it's likely DC Offset.
Just taking a wild guess. Do you have one or more sub-woofers and if so, could it be eminating from them? I have two subs and had a hum problem until I went with Audioquest sub-woofer cables which have ground wire horseshoe connections. You simply connect the ground wire to a grounded screw on the sub. Also consider installing PSAudio Power Ports and other PSAudio filtering products. Just some ideas.
First, it ain't a common feature of prepros but it is a common problem with cable boxes. These are often installed with improper grounds, i.e., remote from the main house ground resulting in a ground loop. Relocating the cable ground to the main ground is a common fix. You can test this by disconnecting the input to the cable box.
Otherwise, normal ground sleuthing is recommended.