I don't know that tapping on tubes will reveal any manufacturing flaw. I have two pairs of Sophia RP's bought here as a slightly used quad set. I use only one pair at a time. I've had them going strong for eight months with many hours and no problem. Mine are used in Wavelength Cardinal Signature XS monos.
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.