You're better off with the 150 meg ohm (or higher value) resistors in that capsule circuit. Go too low on the capsule resistors, the time constant of the capsule and resistors "loading" it falls in the audio range. If you have a number of the 1/4 watt or smaller 100 meg resistors, try two of them in series in place of the 150 meg resistor. The original tube in the M49 (the venerable and expensive AC701) liked to be loaded with 150 meg. There is not a lot of published data about how the 5840, 6072, 5703, 7586, etc. behave when used with high grid impedance. Those tubes I listed have been used in condenser microphones over the years.