I would make two ground wires about 1/2 meter to 1 meter long with alligator clips on each end. I would ground the arm to phono stage and turntable to chassis of preamp or another source. If that does not work then reverse it to see which helps. Sometimes it helps to ground it to the screw on the wall sockets.
I have a Systemdek IV bigger brother to the II along with a Rega RB 600 and I have had a RB250 with a Grado Reference and no problems. Since I prefer a bit tighter bass than most mm types I use a moving coil cartridge instead. But the Systemdek IV RB250 and Grado $300 reference model was a fantastic combination. It's not that much difference between the II and the IV other than electronic speed control and heavier platter, springs, etc. Play with the the wires and alligator clips. It may solve your problems.
I have a Systemdek IV bigger brother to the II along with a Rega RB 600 and I have had a RB250 with a Grado Reference and no problems. Since I prefer a bit tighter bass than most mm types I use a moving coil cartridge instead. But the Systemdek IV RB250 and Grado $300 reference model was a fantastic combination. It's not that much difference between the II and the IV other than electronic speed control and heavier platter, springs, etc. Play with the the wires and alligator clips. It may solve your problems.