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Best resistors  for U47 are both 100M .  60M resistor was used  with 0.5uF coupling capacitor in U47. This 60M-0.5uF version was built for radio broadcasts , it has less low end than 100M-1uF version. Sometimes, 60M-0.5uF version even better: for vocal or other instruments ( natural low cut).  Grid resistor ( as a part of the highpass filter of the first order) and coupling capacitor (as part of a second-order highpass filter) affect on cutting of low frequencies. But, already a grid leak resistor 100 M you have 3 Hz filter cutoff. It is sufficient to apply the microphone in a studio.1G - too high value(!!!!) for most of old tubes. It is believed that higher grid resistor value may increase low end. But in reality,  you will not hear marked increase  low end  in comparison with 100M. Even smallest fluctuations temperature or humidity will cause fluctuations of 1G  resistors value, - it is not good. Of course, you can use laquer for avoiding of humidity influence. But that, huge  grid resistor may also  affect on stability of  work of the tube. Use normal value- 100M, believe me. I tried 220M  grid resistor with a few types of old German steell tubes (including VF14M), for example, and I did not hear any pleasant improvement in sound.