Type "Grado hum" into Google and you will immediately find numerous discussions about the legendary Grado hum, which occurs with some turntables, most notably the Regas. Like many people I had thought that the hum issue was somewhat exaggerated as I had used a Grado Gold1 with a Rega and found the hum produced to be virtually inaudible. This was with the Grado Gold1 mounted on an RB250.
Recently, I purchased a Grado Sonata and mounted it on a Michelle Tecnoarm. Unlike the RB250 the Michelle Tecnoarm is not internally earthed and has an external earth wire that clips onto the earth terminal of the phono stage amp. Set-up like this the Grado Sonata hummed big time, so much so that I was feeling like taking the Sonata off and reselling it. Searching the internet did not reveal any sure fire solutions. Mu-metal shielding around the motor had worked for some people, but not for others.
Earthing the tonearm to the motor had also worked for some, but it didn't work for me. However, while attempting this, I noticed that the hum reduced in volume the closer I moved to my integrated valve amp. So I clipped the Tecnoarm's earth wire onto the metal front plate of the amp and the hum vanished instantly and completely. Even at full volume there is no hum audible.