I upgraded from the Massdrop O2. The AAA789 is beautiful sounding, it must be at the limit of my ears as the sound is so incredibly clean (no sign of noise or distortion even on the very highest setting). There is a nice pop on kick drums which suggests to me quick transients and plenty of reserve power. The sound is precise, clear and spacious.
However I hear a couple differences from my O2. First, I’m operating at the highest level (III) with all my headphones (HE400i, HD6XX, and DT880 [600 ohms]), so less power than my Massdrop O2 medium gain, at least it is off the SE outputs. This doesn't jibe with what they say on this drop though the details report 3.3x at the highest setting, which is comparable to my O2 on the high setting.
Second, with regard to frequency response, this is definitely brighter than the O2. While the AA789 may be flatter than my O2, it means that I now prefer listening on the HD6XX (as a previous reviewer said “What Sennheiser veil?”) and found the HE400i and especially the DT880 to be a bit too bright at first (I used to prefer those 2 with my O2). OK, back to the music...
UPDATE: A couple of days later I am appreciating the clarity even more. Damn this thing is CLEAN! My ears have also adapted to the additional brightness and I enjoy listening on both the HD6XX AND the HE400i. So far, the DT880 still sounds too bright to my ears. The separation and therefor imaging is phenomenal. It works great with contemporary mixes but that means some 1960s pop music like the Beatles is extreme in headphones. I like the auto-off feature, but I wish it also included an auto on (for when I switch on my player via remote).
UPDATE 2: After a couple more weeks, I keep finding new details in my music. I love to sit home and listen. The noise floor is impossibly low (I live on a quiet street in a small town with double pane windows, and I can't hear a thing, even at the highest gain setting).
UPDATE 3: It’s been a few months, got a balanced cable for my HD6XX, which does increase the sound level, but, sorry, I don’t hear more detail. I also picked up a pair of the Massdrop KOSS e-stats. although I still love the detail in this THX amp, I think there is bit more from the Koss e-stats, especially in the highs. However I think there a couple more dBs of lower bass with the HE400i than the Koss. So if you want a bigger bigger bottom end, this is your amp, but if you want sparkle, it’s the Koss. Finally this THX is QUIET, while the Koss has a bit of hum.