Do a search for “THX” in the “SPL Phonitor X” thread, and you’ll find a good number of comments on both from multiple posters.
They sound very different … its one of those comparisons in which it is hard to imagine not being able to tell them apart. I don’t know anyone that’s compared them and come away preferring the THX AAA 789, even when I’ve run blind comparisons at some of the meets I’ve hosted.
Ultimately, though, for me it would depend on the headphones in question. If you’ve already got seriously good headphones (Focal Clear, HEDDphone, Sennheiser HD800S, Audeze LCD-X, ZMF Aeolus or Auteur or better), then the Phonitor X will reward that. If not, you’ll get more bang for your buck going with a THX AAA 789, Jotunheim 2, Lyr 3 or Rebel and spending the difference on getting better headphones.