On every preamp, there are sweetspots and this is normally based on the signal integrity through the circuitry. On the STP, this sweetspot is quite large because of its’ design. You will notice that even at background levels, it will maintain proper image and staging. I can assure you that the majority of other preamps will not offer the same! Despite what I mention above, it is still natural to ‘prefer’ a certain level of any recording as certain instruments or vocals will achieve the desired prominence.
I would say that any setting from 13-63 would be beneficial and the higher the better because the signal would be more dominant with higher level. This doesn’t mean that lowering the level of the DAC would cause an improvement because the loss on the DAC side would be more significant to the gain on the preamp side (it would be a step backwards).