I have the ADI-2 and the THX 789....it's an great combo. But, I would be perfectly satisfied using the RME alone. In my humble opinion, I'd say the 789 makes the background quieter and the sound warmer. I'd describe it as the RME alone is for more critical listening, where every single noise is heard (sometimes that's good, sometimes bad), while going through the THX you can just sit back and enjoy the listen. For example, listening to 'Jesus Doesn't Want Me For a Sunbeam' from Nirvana's Unplugged album, at the end of the song the crowd claps. With the RME every single clap is super crisp and separated, but almost too much so that's it's a little abrasive to the ear. With the THX the detail is still there but the abrasiveness is gone. Maybe some of that could be EQ'd away when going back and forth, but that's simply unplugging from one and going straight into the other. Weird for me to have that opinion since my quest was always for the most detailed sound, and to say that too much detail is detrimental to the sound is not something I expected. It's nice to have the option t listen either way, and maybe the effect is minimized using various headphones (I'm using Fostex TH900 mk2's).
I'd say the RME setup can be as exhausting as you make it. I honestly haven't even tried any of the fine tuning EQ options....I EQ through the source (JRiver on a desktop PC). Although I have played a bit with the bass and treble knob adj's and they work well. The biggest thing I noticed as far as simple "on/off" impacts with the RME were items in the I/O menu, which aren't nearly as intimidating. In fact, the biggest difference I found was the "Loudness" function, which is a button on the remote. Turning it on really brought out the sound detail mentioned above....but again, almost too much at times.
My comparisons are very limited as I only have a Mojo. But I like the capabilities and the sound options with the RME (for example the loudness mentioned above) that are absent in DAC's you simply plug into. But I'll certainly admit that can be exhausting as well, as you're always wondering "will it sound better if I adjust this". But I'd still rather have the option.