Did you try disconnecting any and all sources? First place I would start at is with the TV and cable/satellite boxes if any. Then the Rega and Nad. Last would be the sub. Once you find it's either a source or just the pre and amp combo a little more troubleshooting can be done.
That being I do have a ground loop too. I had dedicated lines installed witch didn't help at all (for the ground loop but sure dropped the noise floor). Then I added a balanced isolation transformer which helped a lot but didn't totally eliminate it but cut the hum way down and changed it to more of a slight buzz. My source is a meridian G08 to a Bryston B100 using rca's. If I lift the ground in the power cord on the Meridian it's totally gone. I have taken the meridian to 2 places and connected it with balanced ic's and hear no hum or buzz.
That being I do have a ground loop too. I had dedicated lines installed witch didn't help at all (for the ground loop but sure dropped the noise floor). Then I added a balanced isolation transformer which helped a lot but didn't totally eliminate it but cut the hum way down and changed it to more of a slight buzz. My source is a meridian G08 to a Bryston B100 using rca's. If I lift the ground in the power cord on the Meridian it's totally gone. I have taken the meridian to 2 places and connected it with balanced ic's and hear no hum or buzz.