There are two main settings that will typically affect sound quality:
This is how sensitive the microphone pick up is and this is adjusted on the handheld microphone, bodypack or SKP transmitter itself. Typically for general speaking the sensitivity should be set around -15 to -18 dB. You will need to adjust this depending on the exact application. Closer to 0 dB is more sensitive and further away from 0 dB is less sensitive. If the microphone is too sensitive the input will overload the microphone and the AF level will peak and the audio will be clipped. The sensitivity should be set so the AF PEAK indicator only lights up during the loudest passages.
This is how strong the final signal exits the receiver into the sound board/PA system/recorder etc. The "AF OUT" setting is found only in the receiver's menu settings. Typically this will be set around 0 dB or +6 dB if you connecting to a line level input on the sound board/speakers/etc. If you are trying to connect to a device which expects a mic level input, you will typically need to drop this AF output down to -24 dB or -30 dB.
NOTE: If the AF Output is too high it may overload the input (peak) on connected device and may create undesirable noise (distortion, rumbling, etc).