With out knowing how your gear is wired, it is hard to say if using the grounding lug on your preamp is "safe."
Why wouldn't a ground lug be "safe"?
I would expect any decent phono preamp's TT ground lug to be "lifted" from earth to avoid common mode interference and not be connected directly to earth.
Huh? Something would be amiss if there's no continuity between the phono preamp ground lug and actual earth ground.
The only way to ensure protection from common mode interference would be to used a balanced phono preamp with balanced connections to the phono cartridge, while maintaining separation between the earth ground and the cartridge's neutral leads.