I have arrived at a "semi-objective" test protocol which, over the years, has served me well to weed out marginal microphone tubes, including those with hidden defects. This will work with most popular specialty tubes for microphones. This article will use the VF14 as a prime example. It is equally valid for other tubes used in microphones, including the decibel references I used.
I have several healthy, low-noise-select, VF14M at my disposal. I install one of them in a perfectly restored, optimally-functioning U47. That mic is inserted into an acoustic isolation chamber which blocks external noise down to -120dB. (You can stuff the mic under a heavy pillow- same effect).
I maximize the monitor level of the headphone output of a special Nakamichi mic preamp I use which also has a white noise generator.
I switch back- and forth between mic noise and white noise, until I have calibrated the pre-ampís white noise output to exactly match the level of the mic's white noise. (If you don't have a white noise generator, you can fake one by dialing an FM radio receiver between stations, and feed that signal into a channel of your board/preamp whose level you then adjust to be equal to the noise output of the reference tube.)
I then substitute the reference/calibration tube in the mic with the tube to be tested. I let the mic run for a day or two, after repeatedly re-calibrating the supply voltage to be within specs for the model. That means ca. 105VDC for a U47, or 210/6.3VDC for a U67, for example. Now I compare the tube's white noise level to the reference white noise level. As my pre-amp is equipped with a dB-stepped attenuator, I can quickly assess the unknown tube's noise floor relative to that of the Neumann-spec reference VF14, represented by the calibrated white noise whose level I do not change in this test.
After all of that, I finally undertake an "in the field" test. From many years' experience I know at roughly what level tube noise will interfere with quiet passages of a recording session in a modern-day, state-of-the-art recording studio. I can extrapolate from that experience what noise would become objectionable by simply talking at moderate levels into the mic at a vocalist's distance from the capsule. There is not much human tolerance for noise from a VF14 or other tube-equipped condenser mic: even an untrained ear will instantly detect 4dB elevated noise from a tube (a trained ear can distinguish as little as 1/4 dB noise elevation, especially in direct comparison to another noise source).
My conclusion and yardstick for selection: Levels of white noise above +2dB from the norm established by a low-noise selected microphone tube will interfere with music in a recording session in two ways: white noise at that level can be heard directly. It also produces masking noise around the audio to be recorded: the sound, especially higher frequencies and percussive passages, will sound slightly muffled, matted, and opaque when masking noise interferes.
In sum, I have established a reasonable cut-off for usability of a VF14 at around +2dB, when compared to a known good, low-noise-selected VF14M. +2dB is also a noise threshold where most people, especially recording engineers producers and artists, will start to notice tube noise from a microphone in general.
If anyone thought that this is a "seat-of-the-pants" approach to objective noise analysis, it is. But then I remind you that a lot of supposedly objective quantification in audio is quite subjective: for example, the decibel was originally defined by Bell Labs by sitting a bunch of people in a room, and have them raise their hands when they start hearing a faint sound fed into the room.
P.S.: To demonstrate how specialized tube noise requirements in high impedance microphone circuits are: I can often install an EF86 whose steady state noise, discharge sputtering and whistling made it unusable in a microphone without any audible noise in my home stereo power amp's first stage of gain: a seemingly defective, unusable microphone tube often works fine in a lower impedance, higher gain, environment.