How to set volume to maximum

Hello all

I just installed Volumio 2.31 on a RPi3/HiFiBerry Digi+ and I am configuring it now.

I changed the output device to HiFiberry Digi and now have a couple of questions before I connect it to my stereo system.

1 - Is there no option for Hifberry Digi+ ?
2 - Should I2S DAC be turned off?
3 - How do I set the volume to stay at 100% (maximum) all the time? (so I control the volume only with my pre-amplifier)
4 - Anything else I should change?

Thanks in advance. Your help will be greatly appreciated.