For mike gain, add an to the mic circuit. Think of this as more along the lines of audio compression than as gain per se.? You will want to put a small elecrolytic cap between this board's output and the mic wire from the mainboard, as that line has voltage on it that this add-on board may not appreciate.? This board has exciter voltage on it's microphone input (since it comes with eletret element itself, that you will remove) so that your line from the hand mic can go right to it.
I chose to include a 50K trim pot soldered right to this board for level control. It makes a hell of a difference in a good way, from signal reports. It takes me from being in the noise, to at least readable copy or from readable copy to "I can't believe you're only running ten watts!".
Give it a try.? I also use one of these boards on the speaker line just before the volume pot. With three selectable gain levels, you can choose how much "help" you want for either application.
Ted