Mine arrived, and I have put a few hours into listening. What follows are my impressions and opinions, and as an objectivist, we both know exactly what those are worth.
Using the Cayin N3 as source, I listened to my repertoire of reference tracks, doing a lot of back and forth between the Cayin's own output, the Chord Mojo and the Monolith THX portable. All listening for this round was done using my Noble K10 CIEMs, mainly because they are awesome, and because the level of isolation they give help me focus on the music. I'll do another run with the HD6xx later on.
Knowing what I do about sight bias and the inaudibility of the minor differences in specs, I'm not going to ramble on about subjective impressions of the sound much. I'll do a little, but if that is what you like reading, you'll want to find something else to occupy a few minutes. I will, however, tell you about what I CAN hear, and what I think of the user interface.
First, and right away, it is clear that the Monolith is not a quiet device. Turning it on reveals a definite hiss, I'd say 15-20dB. This quickly spills over into the music, pushing the noise floor up in quiet spaces in recordings. The Mojo and Cayin do not do this. Otherwise, I'd say it sounds just fine. Not great, but fine. I understand that there is a brain-burn-in effect, and maybe since I listen to the Mojo often, I prefer it and the dead silence it provides when no sounds are required, but in changing back and forth, I found I also preferred the Cayin. I even tried the old trick of turning up the Monolith a bit, and still found it not quite as pleasing as the other two. This is not to say the Monolith sounds bad, it does not. It sounds fine. It's better than my Note 8 by a country mile.
Moving on to the user interface. Well, I think Amir found the same thing....it's fiddly, and if you're not fast enough, the menu closes and you have to start over. Fortunately, there are not a ton of settings to mess with. Once you get the auto input turned on, you can leave them alone for the most part. There are three presets for parametric EQ and high\low shelf EQ settings. DRC (dynamic range compression), yuck, I may play with it someday, but why? Ah, there it is!! The reason I was still excited about this unit, now that we know the measurements are good, but not up to the standards of the Massdrop box using a pair of the 788 chips. Dirac Sensaround II!
Enabling this feature filled me with excitement, and you can imagine my dismay when I discovered that it kind of sounds like I stuffed a little bit of cotton in my ears before wedging the customs home. As Amir noted, it's a subtle effect, but noticeable. Rapidly popping the effect on and off, I found that it does add a sense of distance from the music, but it's weird, and a bit hard to explain. In one of my favorite tracks, Norah Jones is very close to the mic, and I like that. With the Dirac Sensaround enabled, she does not sound as close, and a bit muffled. Disabling it sounds preferable. Now, I'm not saying it's all bad. There are a few tracks, like Punch Brothers - Moonshiner, where three instruments are playing together and the Dirac effect seems to help....maybe....but once the tune gets busy, that effect is overcome by the muffled sound of distance that is just not realistic. It just seems to eat up some dynamics. The closest I've come to liking the effect is with a single vocalist recorded at a distance from the mic. I've not given up, there may be some music that this works really well with, but I've not found it yet. It really did damage to all the classical tracks I tried it with, muddying up the sound and just making it kind of unpleasant.
In summary, with very sensitive IEMs, this is not the HTRF-in-a-box you're looking for. It sounds acceptable, if you don't mind the hiss (which may not be a factor with the HD6xx), and it is definitely more capable than the shoddy afterthought DAC-stage in most smartphones. There will be more impressions as I try it with different gear and music styles, and I hope any others that lay hands on it can chime in with their thoughts as well. At this point I'd say it's an average effort at best. More to come....
Quick footnote: I jumped over to some of my more bass-heavy classical, and found that the Dirac effect simply decimates the lower registers. The booming reverberation of the big pipe in Felix Hell's recordings is just....gone. This is not at all what I'd hoped for.