I have a lot of headphone equipment but couldn't resist this at the sale price with free shipping. It came in in perfect condition, great job Monoprice on packaging. The stock tubes sounded fine and the amp made no extraneous noise I could detect.
I purchased new driver tubes and a new power tube and sound even improved!
I then ordered a 1966 RCA 6AS7G NOS tube and heard a hum. Not sure if it's just a burn-in issue with the tube, it sounds excellent but I can't use it in this amp.
Also some other form of input would have been nice as it only has USB and RCA. There is a 3.5mm input also but I don't use that. No coaxial or spdif.
I like it though and it does sound good with plenty of power.

It is powerful enough for my inefficient 600 Ohm headphone. Replacing the Chinese made 6N5P and 6N3 tubes with a balanced US made NOS RCA 6AS7GT and a matched pair of 5670 improved the sound quality a lot. The DAC works very well . Very happy with the purchase and highly recommended.

I don’t have any experience with Tube amps before this. I generally use solid state amps. I am really enjoying the sound I get from this thing. It does get quite warm, but the more I listen, the more I enjoy it. I was worried when i first turned it on I heard a hum, but once the music starts, it goes pretty dark. I hear the sweet nuances in the music. This is a bit warmer than my solid states. I was worried about how it would sound with my Planars, but so far it has been great. Mainly been listening with my M1060s. What a great pairing, both from mono price. No I was not paid, just a fan. I have tried with my HiFiMan He5se, but I keep coming back to the M1060, they are so clear and clean. Its big and takes up space, but the fact that the tubes are covered keeps me from getting burned.

OK, so I've had this amp for several months now. I've waited to post a review for quite a while.
First off, the bad.
The quality control issues with this amp are real.
I bought mine on Amazon, and when it arrived there was something heavy clunking around inside the unit and it wouldn't power on. The usb input for the DAC on the back was slanted at a 45 degree angle.
Not wanting to deal with an exchange from Amazon, I figured I'd open it up and take a look inside.
There is a larger, approximately 2lb transformer mounted directly to the frame over the DAC. When this unit was assembled, I'm assuming that they never tightened the 4 screws that hold the transformer. During sipping, the nuts works lose, and the tranformer dropped on the DAC. The DAC is mounted on thin, stamped steel brackets so the transformer bent the brackets down, hence the angle for the USB input. The transformer had come unplugged when it fell.
I remounted the transformer, and straightened the DAC brackets, and hoped for the best and powered it up.
After that the amp worked perfectly.
Now the good.
This amp is sounds amazing. No hum or noise at all unless you are using a source with a cheap power supply, or you have some ground loop issues in your house.
It is also the only full tube amplifier that I''m aware of that will drive a full 1W per channel even into low impedance headphones. This amp is seriously built, it has a giant torroidal impedance matching transformer in the bottom. The transformer alone has to weigh at least 5 lbs, this is where most of the weight for this amp comes from. The power stage is all very heavy duty, and all the wiring to the tube sockets is heavy, solder joints all looked good, etc. This amp will drive any tubes you throw at to their full potential. It has also paired well with every set of headphones that I''ve thrown at it without blinking. Anything from 30 ohms to 600. It sounds good with the Chinese tubes out of the box, but I agree with the previous poster that the GE's make a marked difference, and are the best preamp tubes that you can get for this amp without paying more than the amp for tubes.
However, I find that the power tube has more of an impact ont eh spund than anything. I've tried an RCA Octal 6AS7G, a GE O getter, and a GE 5 star 6080 in it so far. My favorite by far is the RCA, it is just a magic combination with the GE pre's. It really tighten up the bass and lowers the bass extension by quite a bit, and also gives it a higher, warmer treble extension. No noise or hum at all, my second favorite tube is the GE 6080 5 star. I didn't care for the O Getter at all, it was much more compressed overall. I actually like the Chinese power tube that it came with better than the O getter. The Chinese tube actually has a nice, wide transparrant top end, it is just a little clipped in the bass region.
The DAC in this is fantastic, I actually like it better than the DAC in my LG G8. It is in the same family of Sabre DAC, s but it is superior to the DACs in the LG phones, and they are the best in the business. The DAC work perfectly with my phone with an OTG driver, or with my computer with the Savi driver. zIt is a pretty neutral DAC and is a great pairing for this amp.
So, at the end of a long winded review, I would have to say that other than the ridiculous process of having to repair a brand new purchase, this is as close to an end game amplifier that you are going to get without a much more substantial investment then even the full list price of this. And if you want tubes, and the ability to properly drive low impedance phones, it is pretty much the only game in town without going to a hybrid like the Liquid Platinum.

Best sound with JRiver Media Center 25,
use driver Asio fro Saviaudio:
https://support.hifimediy.com/support/solutions/articles/8000007400-savitech-asio-driver-installation-guide

mine is plug on USB3.1 type c
use shielded cable for AC input
cable monoprice: Cab-PW-105C-06SH

Ha and just to bad there is no Lighted panel to see the Knob in front.

Pros:
Full tube amplification
Three switchable inputs(DAC, RCA, and 3.5mm line in)
Switchable output impedance to match different headphones
Good DAC.

Cons:
Mediocre stock tubes
Large size
Tube access hindered by bolt on panel(not a real big deal)

I've had this thing for a bit over a month now and it has brought me a ton of musical joy. It sounded ok out of the box but do yourself a huge favor and swap out the preamp tubes(the smaller ones)with GE 5670Ws. I bought a matched set for $22 so it isn't expensive but it makes a huge difference. The GE tubes get rid of a little graininess the stock tubes exhibit and really clear up the whole presentation significantly. You can swap out the Output tube for some gain as well but it's not as dramatic.

This amp is not overly "tubey". It's a fairly neutral sounding amp that really lets you hear the differences between headphones. The included DAC is fairly neutral as well, maybe to a fault. Sounds a little flat in comparison to some other DACs played through the RCA inputs. The DAC worked well with my iPhone as well(using the Apple USB adapter).

This amp is a pretty good deal at the regular price of $399 but keep an eye on it because there are price drops on it occasionally. At the price I paid for it was a screaming deal. I'm very happy with the purchase.

Really improved the sound of my oppo pm2 headphones.great deal.I had a little trouble finding the driver for windows 7.

Great sound. This thing rivals more expensive units, and you may prefer this one anyway as it may come down to preferences in sound &/or synergy with your other gear.The tubes that come with it have a nice 'saturation' point towards the higher settings on the volume dial, which softly compresses the sound beyond a certain level. The preamp function is great, it apparently uses a high quality line input. I connected the 'line in' to a solid state phono preamp output. In this configuration, it acts as sort of a tube buffer giving the sound a touch of warmth, perhaps due to some even harmonic 'injected' into the tract. The DAC function is very solid.I found the 2.8Mhz DSD mode to provide subjectively 'the better' sound (to my ears) as long as your app can upconvert PCM to DSD on the fly.The USB cable included is great, but please do yourself a favor and add a few inexpensive ferrite rings to it (one is already pre-installed on the cable) it will bring down the noise by a precious few db.

The DAC works as described, the preamp out sounds good in either mode. Headphones sound open and full. Unfortunately there is only one stereo output, it should have another output for subwoofer. There is no way to turn headphone signal on or off, there is also no preamp stereo output selector, the headphones and speakers play at the same time so you have to turn your main amp down or off to listen to just the headphones. I replaced the tubes with NOS U.S.A. made mil-spec tubes for about 30.00 $

This sounds amazing! I've paired it with the magneplanar headphones and it's like listening to my music for the first time again!I'd like to connect my iPhone to it as a source but I can't seem to get the Monolith to recognize it. Any ideas?