A problem might be that, for technical reasons, every of RME ADI-2's outputs has a different maximum level, Line Out balanced, Line Out unbalanced, Headphones Outs, (DAC's) IEM Out.
Line Out unbalanced does not follow balanced at/above +24dB reference level, but switches back 5dB when you dial up across +24dB Ref. transition, and back up in the other direction - strange BTW when you use the unbalanced Line Out.
(Not how I had designed it).
For Heaphones Out you already have the "Impedance based Level Meters PH 1-4", see manual page 89, witch auto adjusts the meters.
The current "dBr" reading on the Vol. display, in conjunction with the blue Ref. Level line — +4dBu, +13dBu, +24dBu — (although absolute values only refer to balanced out and unbalanced below +24dBu Ref.) gives as much information as possible.
I must admit that before getting used to all this and reading the manual I was confused with the level and vol. situation and display too.
I think, that's the price you have to pay for the great versatility.
Finally ADI-2 Pro sounds great, extends the usability of my headphones by a large margin, and levels I always dial in by what I hear anyway.