My response is to say: "whatever works". Tracking down hum and buzz problems can be notorious dificult to do, and cure.

In connecting the tonearm ground wire to the earth terminal of the Marantz amplifier means the arm is now earthed by your Primare phono amp via the screening of the phonostage output cables.

Why this should be the solution to the problem I don't know - but as I said 'whatever works'. Now just sit back and enjoy the music.