Circuit design was based on information provided within the following official Kenwood TS-590SG manuals:
1) Instruction Manual, p.3 and p.70.
2) In Depth Manual, p.43.
The 4-button membrane keypad is this: http://grobotronics.com/membrane-keypad-1×4.html
For my MIC, the approximate values of POTs were set as follows:
5k POT set to ~2.15kOhm (typical, keep this close to 2k)
50k POT (upper) set tp ~3.66kOhm
50k POT (lower towards MIC) set to ~ 46kOhm
Other combinations of POT could be used, but keep in mind that you should not restrict the DC current that the internal FET of the MIC needs to properly amplify the MIC sound.
The 20k POT along the audio MIC line adjusts the low-cut freq response
The 10uF capacitor on the Vcc line, used to smooth out any ripples on the +8V DC line from the transceiver, could be set to a lower value (e.g. 1uF) as it de-emphasizes the high frequency audio of the MIC since it is effectively connected in parallel with it, especially when the 50k upper POT is set to a low value.