If the Discrete DAC is a DAC with preamp capability but you already have a preamp in the chain it has a "Unity Gain" setting for it's volume control for use in that scenario. So you set the Discrete to that Unity gain volume setting and hook it up to your preamp like any other source. For example on your Lumin that setting is a 100 but other DACs have different settings.
No I wouldn't as the DAC in use now in both of my systems have preamp capability so they are both set to the Unity Gain setting which in most cases is max to get you to 2v or 4v. I don't know what the difference is for MSB with "native" or "optimized" but I'm sure Mike or some of the other owners do.