Macster, I own a VPI Classic 1 (w/Classic 3 wand upgrade). My "primary" carty is a Sound Smith Zephyr. No hum and not even using a ground wire from the TT to the phono pre. My back-up is a Dynavector 20X2 (H). No hum with that carty either.
I've also used a Clearaudio Maestro and Ortofon 2M Black. Those carties didn't work out for me because of compatibility issues, which may have been my fault. But no hum with either of these carties.
Check Macdadtexas comments above. I tend to agree with him. I think it's the motor and the fix is using a well shielded carty like a Sound Smith or DV. I don't know if Clearaudio or Ortofon carties are well shielded, but as stated, no hum problem for me.
I have read elsewhere that Grados are susceptible to hum, but VPI mounts them as options. Maybe the VPI Grados are special order??? Dunno.