The "gain" controls how much yr pre increases the voltage of the incoming (source) signal. So, you can match it to yr amplifier (in the case of very low level phono signal you may have to increase the gain so the auditory level matches line-level sources -- as mentioned above).
The phase switch reverts absolute polarity, as some recordings or components, have reversed polarity.
The phase switch reverts absolute polarity, as some recordings or components, have reversed polarity.