Here is another thing I noticed.  It shows XLR as "+1 dBu" under the "Reference" column.  It also appears to show the Phones and IEM as "unplugged" (if that's what that means).

The thing is... I'm outputting via XLR, and they are no RCA / Unbalanced cords plugged into the unit.  If "(unpl.)" does mean "unplugged", shouldn't that show next to Line Out RCA.  And should the XLR output actually be showing "+1dBu" on the main screen? 

I'm probably just ignorant to what it's doing, but it would be nice to better know the effect of the differences between the 2 units... especially any "real world / daily use" impact.

Nominal/displayed Reference Levels on ADI-2 DAC refer to the RCA outputs.
The levels on the XLRs are 6 dB (one step) higher than displayed on the device.
The Remote app shows the real levels.

On both the “Pro”-models, nominal/displayed Reference Levels are for both the XLRs and the Rear TRSs up to +19 dBu.
For technical reason Rear TRSs can’t do +24 dBu, so they don”t follow the XLRs on this last step.

On both models there’s no sensing if line outs RCA, Rear TRS or XLR is plugged.
Only all the front headphones outputs have a plug sensing.

I read the numbers / specs... but what is the practical, experiential benefit of the expanded range for the ADI-2/4 Pro?  Is it at low level listening, high level listening... is the ADI-2/4 better at all levels?  I am ignorant to the technical benefits of what that means.

I see that the Auto Reference Levels appear to match from +1 to +19, with the ADI-2 DAC FS not having +24dBu.  What is interesting is the Auto Ref readings on the remote app between the two units is different, at the same volume level. What I'm trying to understand is - What is the practical benefit / difference between the 2 units as it relates to that?  At that "same volume level" on the main dial, what is the output difference / benefit between the 2 units and, most importantly, why is one sonically better than the other (if so)?

On the ADI-2 DAC FS it is showing -5dBu.

On the ADI-2/4 Pro SE it is showing -1dBu.

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ADI-2 DAC FS - Auto Reference Levels
The levels at the XLR output: +1, +7, +13, +19 dBu.

ADI-2/4 Pro SE - Auto Reference Levels
The levels at the XLR output: +1, +7, +13, +19, +24 dBu

ADI-2 DAC / ADI-2/4 PRO have different output voltages (XLR/Cinch):
If the devices are driven with the same digital signal, the ADI-2/4 PRO must be turned down if the same output voltages (volume levels) are required. This is why there are different dB values ("Auto Ref Level = OFF") or dBu Ref Level values ("Auto Ref Level = ON") in the displays or when using the ADI-2 Remote APP.

In fact the only level that is different from the displayed Ref Level is ADI-2 DAC‘s XLR output, which is 6 dB hotter (than RCA).

Of course the dBr values (Auto Ref Level “On“) are different between the DAC and the Pro‘s, as dBr is referenced to full-scale, which is higher on the Pro‘s.

In the recording studio we typically run higher levels than are usual in a home environment.
A lot of home equipment would be overloaded with such high levels, hence the diffrence.
Look, e.g. at Yamaha home audio stuff: most of their integrated amps can just take ca. +10 dBu and would be hopelessly overdriven with professional +24 dBu.