Despite your recommendation of not to tap the tube, well constructed tubes do not ring when tapped.
Sophia Electric tube quality
I wonder if others have experienced poor quality with the Royal Princess 300b tubes. Purchased a new pair and one shorted out in a couple of months in their amp. Sophia replaced it and the new pair were extremely microphonic -- rang like bells when tapped with just my thumbnail. Sophia's response to my complaint was, "Stop tapping the tubes.