Hard to imagine that a snubber cap across the power switch would be missing or broken. But I would definitely check.
More distance between turntable power switch and phono pre might already be the solution. Is the power cable to the turntable very close/parallel to the interconnects from the cartridge to the phone-pre? After all the snubber only minimizes the problem and doesn't neutralize it totally.
If nothing works and you are really “desperate” you can build in an extra switch that only turns off the low voltage connection to the motor. You would lose the warranty but the problem would be fixed. But I would first try disconnecting the motor manually (unplug the cable if possible) instead of using the switch and see if the problem is gone.