I'm in the process of researching my next amplifier purchase and trying to sort out cost effective ways to further improve the sound. IMHO dampening tube microphonics to avoid their amplification makes sense, as a taschenspieler can illustrate by lightly tapping the surface near the tube and say: Do you hear the effect?
Though.. I might be a gullible person that's just been tricked by sleight of hand and in a bewildered moment bought into the following plausible explanation: "The further away the grid is, the faster the electrons are traveling (the acceleration between the anode and cathode over a distance by the electric field), and hence the less electrons it can stop, hence a lower mu. On the other hand, when the grid is close to the cathode, the electron velocities will be relatively small, and the grid will have a larger effect on the electron flow; hence a higher mu."
Well, to confirm "the phenomena" there's always the cheap-o path of DIY and if this just the effect of placebo, it makes perfectly sense to reinforce the disillusion so one becomes immune to critique -> http://www.ozvalveamps.org/microphonics.htm