I want to start off and say that this amplifer is worth more than double what the asking price is.Everything from the sound quality, to the UI, to the EQ options are phenomenal. It has more than enough power to handle most headphones on the market, and a non existent noise floor. Music seems to flow out of a pitch black background. You have a great ability to change the sound characteristics, without harming the quality, with the EQ and Dirac. I’m currently using it as my main headphone amplifer, while I wait for my THX AAA 789 amp to come in May.This amplifier is all around super clean!Pros:•Top notch sound quality•Great UI experience•Great EQ options•Fantastic battery life (upwards of 10 hours of constant use)•Fantastic build quality and form factor•That volume knob!•Ability to charge and use at the same time•Boatloads of unadulterated power•Super low noise floorCons:•No Bluetooth support, without external device•No balanced out-ConnorG

Not sure what the issue with the first review, but my unit was very clean sounding. In fact, the AKM and THX amplification makes for the some of the cleanest sound I've ever heard in an headphone amp. However, I can't really say it is superior to my Topping NX4, which is $100 less, both are really good new portable amps that came out. I tested them with Planars and IEM's, and the THX amp works perfectly. I didn't hear any hiss or issues with it. Just clean, refined sound. No sure if its worth the $275 asking price, but definitely a very nice unit with good aesthetics and sound signature for on the go use.

This unit is not perfect, but it does a lot of things well in comparison to other portable amp/DAC combo units. I normally use an iFi nano BL as my typical daily unit but I have used many other prior (Fiio Q1 mk2, Chord Mojo, Oppo HA2SE, etc.) and this unit is my 2nd favorite overall when everything is taken into account (cost, performance, features, form factor, battery, software, etc).

Pros
--Form factor is nice. A little thicker than my cellphone. They stack nicely on top of one another.
--Build quality is good so far. No defects, chips, dings, etc. Casing and know feels nice but the material on the back feels cheap and plasticky.
--Screen is nicer than I would have thought. Easy to read in all lighting situations and elements on screen are laid out nicely. Anything pertinent is on the main screen so you don't really have to dip into the setting for a quick peak.
--Plenty of clean power. I haven't had any issue powering the Campfire Solaris to the Focal Elegia to the HD 6XX. I had headroom left on the unit so I never felt like I had to max it out to get to a comfortable listening volume. I normally listen around ~80 dB.
--Battery life has been good. I have been getting the about ~10 hrs or so with DIRAC and EQ on every now and then.
--Connecting via USB OTG to android works every time and I never have disconnect issues. Has worked flawlessly with Play Music, Spotify, Tidal and Poweramp.
--Works pretty well as a desktop DAC. Used as a DAC for my THX 789 amp. I haven't had much ear time here but everything sounded clean so no initial complaints.
--The DIRAC feature is hit/miss. It's very headphone and song dependent. Sometimes, I barely notice it's on, other times, things sound way too artificial. And sometimes, it adds more transients and effects that make recordings sound less flat and more 3D/natural. This is one of those things you'll have to play with and see how it works for you.
--EQ and the DAC filters I have not used very much. From my short time playing with them, the effects can go from subtle to really good to really bad quite fast. I haven't read through all the adjustments and how they can stack with one another but the possible combinations is welcome. I did use the shelf EQ feature to add some low-end to my bass light cans (Focal Elex) and lower some treble peaks (Fostex Ebony) which it handled tastefully.
--When the DAC is bypassed the amp performs well on its own. I used the amp section with the DAC section of my iFi nano BL and enjoyed that alternate take on the sound compared to the thx amp/DAC.
--I have not tested the optical out yet as my mini toslink adapters are still in transit. I will update this review when I get them in.

Cons
--Gets warm. Shockingly warm. May be an issue for those that keep in their pocket.
--The volume knob is WAY too sensitive. I could sneeze and it would adjust itself by 30 dB. I have had quite a few accidental volume shocks that were not pleasant.There definitely needs to be more resistance for the knob.
--There is some EMI. I have only noticed it a couple of times but it was not enjoyable when it was happening.
--There is some hiss with my most sensitive IEMs (10 ohms). But, my other IEM the Campfire Atlas sounds just fine at 19 ohms. I never experienced any hiss on my Focals, Sennheisers, or Fostex cans no matter how loud I got.
--Fingerprint magnet. A minor con but more of a nuisance.

Conclusion
Overall, I really like this unit. While it is not to the same level as a portable THX 789 that I was hoping for, it still offers plenty of clean power. The DAC is also nice, supports plenty of hi-res formats, and offers multiple input options and the DAC can be bypassed or used with a desktop amplifier.

The form factor helps with slipping in a pocket and the unit does have a nice weight without being overly heavy.

The hiss issue with very sensitive IEMs is a nuisance. I am planning to follow-up with Monoprice to see if my unit is faulty in that regard but otherwise, I don't have any major gripes with this unit. It works well. Feels well. Sounds good. And isn't terribly expensive and offers some nice additional features and integrates well with my desktop setup.

As far as portable amp/DAC units go, you could do a whole lot worse for more money but I think this unit gets most of the higher end offerings correct while coming in at a more budget friendly price. I could nitpick more and be tough on this unit but when you're comparing it to its competitors, this unit gets more right than it does wrong.

It is OK with IEM's but sensational with most big, over the ear cans I have. Particularly with the Monoprice's own M1060. No, I'm not paid for this, I guess this is what they used when developing this DAC. It is super fast, starts and stops without overhang as the music flows, silent background, not the velvety dark type, just nothing type, like pure air between the notes. The best part is the space and the details. I haven't turned the Dirac on yet, no need for it. Tonality is nice and neutral with huge dynamic drive at high volumes without loosing its grip of the music. It is the kind of equipment the glues you to your system because you want to hear every music over again for the way it recreates the recordings from zeros and ones. Beats the Mojo hands down, the entertainment value is right there with some real big boy home DAC's. I've tried it with the LCD X, T1, H6XX, and the 701 too. Yes, the form factor is not the fanciest I have seen, but I could not care less because the way it performs. Again, just OK with IEM's, however, it drove the Isine 20 nicely. If the faux leather on the the back was genuine toad skin I would still keep it! It is that good. And not just for the price.

With my sensitive IEMs and no media connected, turning this amplifier on creates a steady background hiss. This sound remains at the same level regardless of volume, so is additive with the noise floor of the recording, obscuring detail and making for unpleasant listening. It is loud enough to be heard with less sensitive headphones at higher volumes if the track you're playing is quiet and has a non-zero noise floor.

The user interface is fiddly, and testing settings or setting EQ is painful because of the number of button clicks it takes to get to the dialogs. Also, the menu resets itself quite quickly.

The USB ports don't quite line up with the holes in the case. This means it is hard to insert a USB cable.

The Dirac Sensaround II feature is not impressive. It definitely degrades the sound quality, making classical tracks sound muddy and dull. I had high hopes for it.

Finally, the back of the case is leather-like and slightly convex so it will not lay flat, instead spins easily when sitting on my desk.

I've begun an exchange process because I can hardly believe that this device was designed to behave this poorly. If the replacement unit is better, I'll update this review, and if Monoprice treats me well during the exchange, I may even say some nice things.