tl;dr: Just buy it. I agree with the sentiment of not bringing price into the equation. This is a good amp at any price.
I finally got my hands on the beast and have been able to listen to a couple pairs of cans. I've basically been lost in sound since I first plugged it in. If I had to pick one word to describe what I'm hearing, I would give it: Effortless. This is a clean clear powerful amp. It is fast and responsive and drives both high impedance and low impedance cans with ease while maintaining a deep dark background at levels you might actually listen at (yes, if I use sensitive cans and crank the gain and volume all the way up, I hear some quiet humming). This amp sets a new reference for what I call musical without messing with frequency response.
AKG K267 listening notes (Gain I, SE): These are the punchiest cans I know of, but I haven't felt them quite like this before. I can feel the kick drum beats hit my ears with more force than I've experienced on other amps. These cans are great an finding the flaws in an amplifier. If it is thin and lifeless, they will show it, and this amp isn't that. That's my biggest fear about the amp alleviated. Another area these cans shine at is finding harsh highs. They can get downright shrieky or grainy with some amps. Not on the 789. I hear nothing but thick, full bodied, richly textured music and the high end is gloriously smooth. I haven't had this much fun with these cans, probably ever... There's some sibilance to be found on Maximum the Hormone: Rokkinpo Goroshi, but that's a recording flaw in my estimation. The high end on that album is out of control and can be painful on some amps. It's not bad here but definitely a touch high in level.
Sennheiser HD 6XX listening notes (Gain II, SE): Not surprisingly SE on this amp leaves something to be desired. Specs show that it's underpowered on SE and listening confirms. The high end is constricted, and the low end loses punch compared to my standard amp. Not really a fair comparison though. Still great sound, but not what I've come to demand from these cans.
Sennheiser HD 6XX listening notes (Gain I, Balanced): There's that clarity and punch. We're back to 95% of my current daily driver. I don't want to negate the 789 for that last 5%, because if you've never heard it, you won't miss it and I built that amp specifically to be able to overpower these cans. The imaging is spectacular though. Connect.Ohm's 9980's high end crackles/pops are crystal clear and easy to pinpoint. I don't know that my daily driver resolves them quite as clearly. This amp is so fast! On Day 2 I've moved to no switching back and forth between the two. I no longer feel like I'm missing the high end air although I know I would get more of it if I switched back. The point is, it's really not a huge loss, but there is more to get out of these cans, at least in my opinion. Great dynamics. Clarity is retained when the music gets soft. I'm in love with Cassandra Wilson's Another Country all over again with this setup. The percussion is clean and easy to pinpoint each part and she is right there in front of me. Not in a recording, but in a room that I'm not actually sitting in. That's a great track.
Hifiman HE-4XX (1st drop edition) listening notes (Gain III, SE): *** Edit: I stand corrected. These settings work well for these cans. I still don't have a balanced cable for my 4XX and probably won't get one since I won't often be driving it with this amp. It's good to know that the option is there to use them with this amp if I decide I want to though. I've got Billie Jean playing now. I don't know that I've heard Michael Jackson's voice this clearly before. It's always praiseworthy when a new piece of the audio chain can bring out something you've never noticed on a well-worn track like this. I do wish the high end wasn't quite as overpowered on these cans as it is. The glaring cymbals take a lot away from the overall presentation. On to Human Nature (next track, same album), I don't know that I've ever noticed the almost sigh-like inhales in the vocals at the beginning of the track before either. I'll switch it up a bit with some Chunk! No, Captain Chunk! to check out the low end impact and general crunchiness: I don't get the punch out of these that I would expect. The speed and clarity are all still there. I'm guessing using the balanced output would do wonders for these.
Various random IEMs and other more obscure headphones listening notes: Just buy this amp. It is fantastic. It breathes new life into everything I've listened to with only 1 or 2 exceptions.
Audio Chain notes (selected, like the cans detailed, mostly because these are all items that have been or are available on Massdrop with 1 exception, the phone): Galaxy Note 3: Onkyo HF Player → Topping D30 → 789.