Good things come to those who wait
Surface-level stuff first, it's got that classic robust Massdrop build -- aluminum chassis, black finish, sharp angles. Solid, if workman-like. The new finish seems like a pretty big improvement over the old finish they used to use which was more prone to looking greasy over time, this finish maintains its clean matte appeal much better.
Features: Outputs are 'Balanced' 4-Pin XLR, 1/4", and 1/8" -- almost everything you could ask for. Three gain stages and a straight-forward, no b.s. knob to change it. A power button that you press once to turn on and once more to turn off (the button doesn't stay depressed, which I surprisingly like quite a bit more than the alternative) and an input button to change, well, the input. 'Balanced' input, single-ended input, and a through-put that works even when the amp is off (a big plus). A nicely-sized volume pot with a nice knobfeel that could maybe offer a little more resistance, but that's niggling. An auto-off feature, which is also useful.
Sound: Robust. Clean. Ample. Smooth but not smoothed over, refined but not overly polite. Neutral is in the eye of the beholder, but frankly, this is neutral.
There doesn't seem to be any difference in tonality between the SE and XLR output, which is not the case much of the time with a balanced topology. Music out of the THX AAA 789 just basically sounds the way it's supposed to sound, for better or worse; no frills, no embellishment, no adaptation. Other amplifiers -- such as Massdrop's own ZDT Jr., which is a great amp in its own right -- perhaps provide a sweeter timbre in the mids, relatively speaking, or a greater sense of soundstage... but I guarantee it'll be at the expense of some of the virtues of the THX AAA 789's strengths: for example, the ZDT Jr does not possess the 789's control in the low-end, nor the deeply holographic way it articulates complicated passages of production. The ZDT Jr is also a suprisingly clean, neutral amp, but its presentation is a more live rendition, where the soundscape is one seamless whole with pepperings of detail here and there; with the 789 you can pick out precisely where the audio mixer placed each element of the song, and the details pop out at you effortlessly.
I have heard comparisons made to the JDS Labs Atom, which also measures very well -- I haven't heard that amp yet, so I can't comment, but I will say that I've heard the Atom sounds somewhat clinical, which is not the case with the 789, which (surprisingly, to me, not being a measurement-junkie by any stretch of the imagination) just about strikes a perfect balance between euphoric musicality and straight-up precision.
I frankly did not expect to like this amp so much. At the asking price it is a can't-fail, no-brainer of a purchase, if you're in the market for an 'end-game' worthy amplifier that can, if you want, take at least one extra step of consideration out of your chain. Obviously, there is a wait involved, at least if you aren't willing to spend possibly a couple hundred extra to get one immediately: but, much to my surprise, it is absolutely 100% worth the wait.
Would recommend to a friend.