I recently received the THX 789 AAA from Drop and have to say it's not my favorite pairing with the AFC or Ether Cx after a few days of testing.
The THX sounds very clean with dynamic headphones with good dynamics and bass response. It's actually very impressive what this amp can do for the MSR7 in terms of layering of the bass, increasing the soundstage, and making an already fast headphone sound even faster.
However with the AFC the dynamics are very compressed and the mids and mid-bass sound very thin with the overall sound signature leaning towards a fatiguing brightness. I was running the AFC off the "balanced" XLR output, and the power output didn't seem to be a problem. These headphones can get very loud even on low gain from the THX. However, the bass response and transients seem muted or tamed relative to Jotunheim or Lyr 3.
The THC / AFC pair was actually pretty good for very quiet music like piano solos that benefit from a larger soundstage and a low noise floor.
The detail retrieval and sound stage are quite good with the THX / AFC pairing, but I found the overall signature a bit too sterile. There was something missing in the mid-bass and lower mids that caused me to keep increasing the volume, but that resulted in an brighter sounding result. The bright sound signature of this pairing was somewhat improved by using the iDSD micro BL as a pre-amp into the the THX to regain some warmth. However, I still far prefer the Jotunheim or Lyr 3.
I decided to give the Gillmore Lite Mk2 a try for a work setup and will report back when it arrives.