The ground of the tonearm needs to be grounded to the preamp. If you look at your connector box, the RCAs are insulated from ground to prevent a ground loop between the signal and the tone arm ground. Sounds like you missed the connection that comes from the tone arm ground.
Tone arms, contrary to popular opinion, are balanced sources, which is why the arm ground is a separate hook-up when you run RCAs.
Tone arms, contrary to popular opinion, are balanced sources, which is why the arm ground is a separate hook-up when you run RCAs.