I am not sure what your lesson was. How would you know the grid bias resistor had a loose connection? You were just unlucky.I
I once blew a Takatsuki 300B and then realized that one of the grid bias resistors had a loose connection. When the amp was moved, the bias got disconnected and the anode current surged, blowing the tube. A painful lesson.
I replaced the Tak with the new WE300B. The WE are definitely much quieter, since my amps drive my 112dB sensitive midrange horns directly. In fact, the noise voltage does not get below about 3mV using the hum dinger with the Tak, but can get to unmeasurable level with the WE. The bias is also much more stable and I hardly ever need to adjust the bias current. This shows the quality of the construction.
I am not debating the audio quality or the construction of the WE300b merely pointing out that they did not honour their warranty in my case.