I had the opportunity to try two MXL 990, an original and one with a capsule of RK47 in Microphone shares. I'm not sure if the circuits are the same, first, I have not made the change myself, and then the MXL series may be different. The capsule being 90% of the sound, I'll assign the RK47 90% of the results
Both pickups were tested at the same distance with the same preamps, cables identical to the same level. I recorded a voice, an acoustic guitar, rhodes (with amplifier), an acoustic bass, and even a single battery - with a single microphone, really old school!
Note that the MXL-modified has an output level slightly lower but the signal-to-noise ratio is higher. At the same level, the MXL is much louder with a slight constant noise around 50-70, as amended MXL did not.
Better transient response. To 200Hz, the original MXL has a bump (like a proximity effect) while at the same distance MXL-modified is more accurate.
At the bottom, original MXL is not very accurate, an acoustic bass seems to lack definition, body, she finds the capsule RK47.
Best "open". The MXL-mod is much better defined in the top of the spectrum, with more air. The MXL-mod, the voice more breadth, it collects more sound environment, the room. There are concerns with this sibilant voice and the original MXL, which are greatly reduced with the RK47 capsule. The sound is darker but not in an unpleasant way. Say treble in the original MXL seem in comparison more "unpleasant" present, which require eq. With RK47, the voice is simply placed in the mix with very little editing.
The original MXL is a microphone a little mat with serious defects, which requires a lot of rework.
RK47 capsule radically change, it becomes a microphone that has an identity, a tone a little darker, softer 5-to 7 kHz, that I will use for acoustic willingly taken in stereo for live shot with a wide dynamic range, voices, short everywhere we want to try a U87.