SD, let me see if I can give the "objective" pro argument for why tube damping devices MAY make a positive difference. You note that if a preamp is well-isolated this should take care of any potential problem. The mechanical aspect can be handled with good rackage and footage. But how can you isolate it acoustically except to place it in another room (too inconvenient)? This question relates to how loudly one wants to play their system. If someone really blasts it with a microphonic tube in a high-gain block, there is little doubt that a small amount of feedback could be induced with audible results around the resonant frequency. But then, why not just select tubes that don't ring in the first place?
