All tubes are microphonic, but some tubes are more microphonic than others. And it makes a difference where in the amplifier circuit the tube is used.
A severe case of microphonics is certainly bad. However, it may be the case that a trace of microphonics contributes the "warm" sound that characterizes tube electronics.
A severe case of microphonics is certainly bad. However, it may be the case that a trace of microphonics contributes the "warm" sound that characterizes tube electronics.