Gents, my diatribes were only in regard to the OPs question regarding resonance. Obviously adding mass further out from the fulcrum is the most effective use of mass, but (as I described) is problematic for mechanical reasons and is not the most effective method of reducing resonance amplitude. Calculating the change in resonance magnitude will require additional information not provided.
Now if, as lewm proposed, we are allowing the alteration of other factors including "damping, stiffness, materials used" then all bets are off. I hadn't realized that we were discussing a tonearm with an infinite k value. 😁