I know everything about these settings, do not worry.
I think it would be strange if somebody would buy some unit, costing 5000 USD and would not know how to make use of it.
The problem here with the gain is that it starts at rather high level even for MM cartridges- 56 dB. Most phono preamps have 40 dB gain for MM, which is reasonable. With XP17 and the Passlabs X1 preamp and XA25 power amp I have now the situation that the music is rather loud even at position 01 of the volume control of the preamp. I like to listen to jazz music at low levels late in the evening and this is really a bit of a problem.
I was thinking that maybe with the gain switches if you leave all three in 0 position, there will be a lower gain, but nope…