"Alastair is correct in that mass balancing can solve some natural frequency issues but its effect is simply that of a "larger" force being needed to induce the same resonant displacement. Qualifier: this is not necessarily true in all cases. Most structures resonate at very low frequencies, thus if you can lower the natural frequency below the excitation frequency, air loads, or possible engine vibes then this will fix the problem"

This makes sense and confirms that mass balancing can cure flutter. Remeber we are working with an existing design that prevents unlimited alterations. If I were and F90 designer I'd have specced dual MG servos for each surface - that'd be sure to stiffen things up a la Boeing...