Spent a whole day today trying to get rid of this. Pulled all the cords, and opened the AV8. Connectors were seated, but the wires were bunched against a side wall. Rerouted the wires, put a techflex sheath around them for mechanical protection, roughed up the sheet metal by the grounding contacts, and re-assembled the unit. Hum was reduced, but still there. Pulled the bottom off my Thorens, resoldered what wires that looked sketchy, and re-assembled. Even better, but still a hum.
Unpacked a new project speedbox, and tried it out. Hum is significantly reduced. There is still some hum though when the old walwart AC transformer is connected to the circuit, even when that transformer is not connected to anything. And if I plug/unplug my frig the hum is affected. There is no hum when I run the turntable through the Rotel pre back in to another input. It is now dead silent, even at max volume. The internal Arcam board has just a touch of hum, more like a very low level purr. Only noticeable at high volume.
The music from the Arcam board sounds good, I think it is a touch better than the Rotel. But at higher volumes there is that purr. I suspect the Arcam phono board lacks sufficient filters for the transformer hum. Bummer.
I guess you gets what you pays for.