Very well broken down, Twl. I agree about the fundamental aims you suggest. Your analysis of the probable dynamic response limitation of a fluid damper reinforces the reasoning behind Mr. Townsend's implementation I was expounding on above.

But I must point out a flaw, or at least a simplification, in your characterization of mass-damping as a "static" system. Assuming negligable bearing friction (and this might not always necessarily be the case, but for now I will assume that a premium bearing's friction will fall below the level where it would play a larger role in dynamics), the horizontal mass of the tonearm does not entirely resist the arm's deflection by the cantilever, as you state, but conforms to the basic principle 'for every action, there is an equal and opposite reaction'. In other words, the motion of the stylus will produce deflection in the cartridge/tonearm which is inversely proportional increasing mass of the cart/arm (and also to increasing cart compliance), but this delfection will not = zero, just as in the fluid-damped example. (Had it = zero, you then would have been implying that there existed some 'mass threshold' for deflection, which your unmodified arm hadn't yet crossed, but that had been crossed once you added the extra mass. Of course, and again assuming bearing friction isn't mucking the system up at this point, this isn't the case; there is no 'mass threshold' - it is a continuum, and you have simply moved down the scale in the direction of less deflection.)

What the levels of allowed deflection would actually be in each example of these respective damping systems is unknown to me, and I would guess is likely to remain so. Obviously, in each case there are many variables to be played with that would affect the answer. To achieve an optimum balance of dynamical attributes for any given cart/arm combination, the fluid system allows for relatively easy adjustment via modification of the fluid level, so the mass-damped system should probably feature a method whereby the user can either change the amount of applied mass, or more likely adjust its distance from the pivot point, like a common counterweight adjuster, either by sliding or helical means. (Twl, did I just suggest a major price increase? :-)

But I must point out a flaw, or at least a simplification, in your characterization of mass-damping as a "static" system. Assuming negligable bearing friction (and this might not always necessarily be the case, but for now I will assume that a premium bearing's friction will fall below the level where it would play a larger role in dynamics), the horizontal mass of the tonearm does not entirely resist the arm's deflection by the cantilever, as you state, but conforms to the basic principle 'for every action, there is an equal and opposite reaction'. In other words, the motion of the stylus will produce deflection in the cartridge/tonearm which is inversely proportional increasing mass of the cart/arm (and also to increasing cart compliance), but this delfection will not = zero, just as in the fluid-damped example. (Had it = zero, you then would have been implying that there existed some 'mass threshold' for deflection, which your unmodified arm hadn't yet crossed, but that had been crossed once you added the extra mass. Of course, and again assuming bearing friction isn't mucking the system up at this point, this isn't the case; there is no 'mass threshold' - it is a continuum, and you have simply moved down the scale in the direction of less deflection.)

What the levels of allowed deflection would actually be in each example of these respective damping systems is unknown to me, and I would guess is likely to remain so. Obviously, in each case there are many variables to be played with that would affect the answer. To achieve an optimum balance of dynamical attributes for any given cart/arm combination, the fluid system allows for relatively easy adjustment via modification of the fluid level, so the mass-damped system should probably feature a method whereby the user can either change the amount of applied mass, or more likely adjust its distance from the pivot point, like a common counterweight adjuster, either by sliding or helical means. (Twl, did I just suggest a major price increase? :-)