I'm not sure how your iFi handles DSD (DSF) playback of tracks. I've had and currently use DAC's that must be set at 100% output for playback of DSD. Some DAC's have two modes. One mode is preamp which allows volume control (0-100). The other mode is DAC mode which eliminates the volume control and sets it fixed at 100% output. This is the preferred mode for DSD (DSF) playback.
With the Crack having unbalanced RCA inputs, a DAC will usually feed it 2 volts when in DAC mode (100% output). A balanced XLR input for comparison is usually double of that at 4 volts.
I just wanted to make aware that when playing DSD, 2 volts minimum may be the only option and that doing the resistor mod is the only option for more range control.
Personally I run the DAC straight into Crack "DAC mode" and the volume rarely goes past 11 o'clock, but none of the signal is absent ....... that is the important thing, to have all of the signal there.