> Will i hear a diffence AKG 712 Pro or will i need to upgrade to the more Expsive AKT K812 to even hear the diffrence. For example when playing music or watching a movie?
You won't hear the difference in normal volumes. I have the ADI-2 Pro and my employer just bought a UFX II after I requested. I tried to ab test between the two, using three popular phones with different impedances, and I could hear zero difference at normal listening volume. THD+N specs of all RME devices are all beyond human hearing range so they don't really matters.
The ADI-2 however, is able to drive those phones to absurdly high volumes .
So, as long as your Babyface Pro drives your headphone loud enough, there won't be any difference.
> How much better is the DAC inside the ADI-2 DAC FC compare with Baby Face Pro?
as is explained above. the DAC quality exceed human audibility limit for all RME products. Better chips have better specs, but you won't be able to tell the difference.
ADI-2 and Babyface are just devices for difference purposes --- their user interface is completely different. For one you have to do almost everything via TotalMix, good for studio recording, for the other all functionalities are exposed in the menu, good for music appreciation and mastering. Take crossfeed and loudness for instance, they are not available in Babyface Pro. Both are killing DSP features for headphone/monitor listening.
Apart from what ramses listed, the most handy feature is the remote control. you can sit 5 meters away from your monitors and still be able to control the device.