After posting my earlier comment on turntable grounding, I realised I had not mentioned anything about direct drive turntables. (I only restore belt drive types) Early on, I did notice that direct drive turntables were more prone to hum. In some cases, the hum increased as the cartridge traversed toward the centre of the record. This type of hum is induced from the motor itself and is very difficult to cure. Some types of cartridge are more prone than others. I suspect that newer direct drives are somewhat better in this respect than the older ones that I was repairing at the time. (Better platter design to shield the motor) It is one of the reasons that I prefer belt drive tables as the motor is further away from the cartridge, but in saying that, I have repaired some belt drive turntables that also exhibited increasing hum as the cartridge traversed the record toward the centre, and hence close to the motor. Early Regas were notorious and I never heard one that did not hum at least a little bit. They all seem to rumble too, but that is a different issue inherent in breadboard designs. I think Rega has addressed the issue by changing the motor, I'm not sure as I haven't seen any late models in my shop.
But getting back to direct drive TT hum issues, you can try different cartridges as some are more prone to picking up hum than others. I think there could also be a case for installing an IEC socket with RF/EMI filter, or some other type of external filter. It can't hurt anyway. Adding a ferrite to the cord may also help and is cheap. Other possibilities to look at are the internal shielding of the tonearm wires and the judicious use of aluminium foil where the wires exit the tonearm (keeping it well away from mains voltages of course!) I have never tried this, but it may help in chronic cases. If people are into modding cartridges by installing them into new bodies, perhaps a body made from aluminium might shield the cartridge better? A lot of cartridges are made with plastic bodies that offer little shielding. Try one with a metal body and see if that helps. If it does, then you're on the right track.
As audiophiles, we all hate hum!