I’ve only had this for a day so far, but I’m going to write some /very early/ impressions. When I first got the Mystique V3, I wanted to make sure I could really hear the differences, so I listened to a playlist I knew really well on the Exogal, then swapped DACs, and listened to that same playlist on the Mystique V3.
I /do not/ recommend doing this, lol. Give the Mystique some warmup time before listening to it; it was pretty treble-y at first and it’s relaxed considerably since that initial listen.
However, even without any warmup time, already the separation and the way it presents space were immediate standouts. I wouldn’t necessarily say that the entirety of the space is wider/deeper/etc, but it’s more that all the individual elements of songs that felt two-dimensional before feel like they exist in space now. Things that are mixed to be front-and-center still feel close, but they’re more roomy.
Low-level detail is easier to pull apart in the way that you would expect: it’s easier to hear individual voices in harmonies, resonances and reverb are more clear, other busy things like drum rolls are more distinct, and background elements that were muddy before are easier to identify. That said, it doesn’t feel like an analytical listen: sounds still very much flow into each other.
My brain kind of interprets the sound as almost having a sort of V-shape: the bass is fuller and punchier, and while the treble isn’t sharp or shouty and it’s calmed down a lot as it’s warmed up, it’s still sounding a little bit treble-y to me.
Okay, on to some more logistical stuff:
I’ve gotten away with small DACs up until now, but the space taken up by Mystique V3 is probably the equivalent volume of three Exogals or Bifrosts. It’s surprisingly larger than it looks in photos, and it just barely squeezes onto the rack that I’m using for my main setup. On the other hand, I was told the package would be 30 lbs and it didn’t feel quite that heavy when I received it.
The Mystique V3’s output is hotter than I’m used to dealing with, so I’m currently running my amp below 9-o’-clock, though it’s not quite as bad as what @hifiDJ was dealing with. (The MYSPHERE and Diana are less sensitive than the Utopia.) I might end up running single-ended from the Mystique to the Allnic since it gives me a little more volume play, but we’ll see. (I mostly bought the balanced version for the resale value; I don’t see it as a waste to run it single-ended.) Or I could get those same Shure attenuators, but at least for now, volume isn’t being a huge problem.
For what it’s worth, this also adds some unreliability to my impressions since my control over the volume is less granular, and while I’ve tried to match it by ear, it’s pretty unlikely I’m listening at quite the same volume that I was listening to previously. (The Exogal Comet+ has a built-in volume control, and I used to run it around 85/100 with the Allnic.)
The chasis is warm to the touch, but it’s nothing that would ever melt anything or hurt anyone. The RCA jacks seem really high-quality compared to the other connections, so the physical design really feels like the Mystique V3 wants to be connected over S/PDIF and single-ended. I’ve tested both S/PDIF and USB inputs, and XLR and RCA outputs, but mostly just to cover my bases and make sure that they work, not enough to reliably give impressions as to the differences between them.
How I feel about things so far:
Having the most expensive component in my setup be a DAC is kinda weird. I spent nearly $4K, which is basically the very top of my budget atm, and then I put the Mystique in my setup and didn’t just immediately fall in love, but such is the way of DACs. Listening to DACs is kind of a slow process, at least for me.
But with more listening time, the presentation and the space have just really knocked it out the park. The way it presents sound is wonderful. That said, I also knew going in that this DAC likely wouldn’t quite match my preferences, so as I look at source and cable upgrades going forward, I’ll probably try to see if maybe I can tilt things a little less warm and a little more towards the clean/technical side of things, but so far, I like it a lot. 
For reference, my full chain is:
iPhone 11 Pro – (USB w/ Apple adapter) → Singxer SU-2 KTE – (S/PDIF) → Mojo Mystique V3 – (XLR) → Allnic HPA-3000 GT (RCA tubes) → MYSPHERE 3.1 / Diana V2
(cables are mostly all AudioQuest Carbon and Mackenzie)