Phono stage is the source of the hum. It is in my new system and the phono stage is a PS Audio Stellar. The ground wire is secure between the TT and phono stage and there is no hum if the Sugar Cube is powered off.
Is the Sugar Cube, phono stage and line stage all plugged into the same AC line? That would help eliminate a ground loop and may make the TT/phono stage ground wire unnecessary.
If the hum only occurs when the Sugar Cube is powered on, I'd suspect it rather than the phono stage.