Test, test, test... Capsule, tube, TX, everything interacts. The UTM C12 TX, for example, sounds noticeably bassier than its much smaller 3U counterpart. I prefer the 3U here... It's a really great mic to try out different values on. Many engineers use 47uF, 100uF ...as the cathode capacitor on original GE6072 tubes instead of 20uF. The presence of the mic changes. That's the benefit of building two "exact" mics in parallel. Same goes for the DC block 3uF capacitor. It's too high in my opinion, too much bass boom comes through. Depends on each component for sure, Beesneez capsules vs. others... These are typical 251 values used by the mic gurus and most work pretty well.
But first I'd play around with the grid-to-ground resistor on Geeks Mic. It changes the character. It forms a HP with the capacitance of the capsule and sets the corner frequency lower with a higher resistance value. Capacitance of capsules varies a lot, from different manufacturers to the CK12 style capsule itself, I have measured from 40pF to 70pF... that means corner frequency from 130Hz to 70Hz at 30Mohm g-to-g, it's like night and day... Mids, treble, presence of the signal, everything changes. You need to adjust this....
Personally I wouldn't go below 160V or at least 120V for testing with the blocking capacitor. Heatup, tube conductivity, ...
