I have used this amp for about four months now. My dac is a Modi 2 Uber using FLAC files. The headphones I currently own are the Audeze LCD-2C, LCD-X, Sennheiser 6XX, 58X, Hifiman 4XX, and Fostex TR-X00 Ebonys. I have balanced 4 pin XLR cables for each of these headphones and have used them in both modes.
I can say that after all these months, I'm disappointed in this amp and wouldn't recommend it to most people. It has a darker sound signature then say a Schiit amp that I have used in the past. Some might say "smoother", but that problem is that the 6.3mm jack was an after thought and really lacks power. You can also hear distortion at higher levels on the Audeze models and the Fostex X00's. If your going to use the standard 6.3 mm jack then I would only recommended it for headphones that are really not very resolving. It doesn't even sound that great with the 6XX, but it is also not a good sound signature pairing.
Using the XLR does give the "sense" that it sounds better with the increased power. The problem is that you start to hear that it is just not very "clean" sounding. My cheap Schiit Magni 2 that I have laying around sounds cleaner. I just ordered the SMSL SU-8 to see if running the dac in fully balanced will do anything to help the sound. If it does, then I will update a star. The bottom line though, is that you can do better for cheaper and it is not a good pairing for the headphones that I own.
Update: Just got a new pair of Denon D7200 headphones. These sound really good on this amp. Treble is really extended and can be a little too bright on my Magni 2. However on this amp, it takes off just enough that it is not as bright. Unlike my other headphones, this is a really good pairing.
Update 8/25/19 LCX vs. THX 789
I got the 789 two days ago. I have logged in about six hours now. I don't claim to be an expert, but I will try and describe the differences that I'm hearing. There is a noticeable difference in the clarity between the two amps. The distortion I was hearing on the LCX is gone on the 789. I can also hear a noticeable difference in neither the dynamics or the ability of the amp to separate the right and left channel on my two higher end headphones, the Denon 7200 and LCD-X. The best way I can describe it, is that It sounds more three dementional or has another layer of separation to the instruments. The LCX sounds more congested in its presentation by comparison. This amp really is worthy of the hype it has received.