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?

I don't write many reviews, but felt compelled to do so based on the small number of reviews on this, and that they were mixed in nature. First a disclaimer: I'm not (yet) an audio snob. I have not listened to true hi end stuff before. I have MD 6XX, MP1060s and an Objective amp/dac. I listen to MP3s (300kb+) not DSD. Ok, it's out there.

I have only listed to this thing a few times, having just gotten it...but wow, I love it. The soundstage is huge, the sound is huge...it's lush. I did not have hum issues. This felt like a significant upgrade to my objective amp/dac. I have always been a "spend half for 95% of the benefit" kind of guy....I don't want to pay for those diminishing returns. Which is why I picked this...seemed like a reasonable price for what it was. I did not expect to like it this much., so now I'm wondering, "what IF I spent even more??" Oy.

Oh, and be aware that this sucker is almost unimaginably heavy for what it is. Back in the old days, audio quality of a given device was based on weight (lol)...if that still holds today, then this thing is a gem.

Good as tube amps that cost much more.

I've done a head to head using HD800s headphones on a few amps which it outperformed and my Woo Audio WA7tp with high end NOS tubes and this Monolith sounds just as good straight out of the box. Incredible value. Interested to look into tube options for this.

First: This review is 100% genuine with no expectations on arrival.

OK, I will admit, I really wanted to like this headphone amp. It checks all of the checkboxes I want to see in an all in one unit. The short review is that I think (total guess) that they rushed the design based on demand. The depth of sound, the power to the headphones and the quality of digital decoding - that is not an issue. In fact, if I ignored the issues with this unit, it would be a COMPLETE WIN. Issues listed in the Cons section, below. An important note: I returned the first unit and have similar issues with the second.

Pros: Depth of sound, power, decoding quality. This is an ambitious project for the Monoprice folks, no doubt, and they were sooo close to pulling off the deal of the century. It has all of the right ingredients.

Cons: The cons are in two parts: Hum and Wiring

Con 1: Hum. Hum that never goes away. I rolled three other power tubes in this unit, a couple of which did reduce the hum, but not enough. Even a few hours in on the Chinese power tube, the hum effect was reduced, but it did not go away. The hum is unaffected by volume. It sounds like an EMI issue - there are two transformers in the unit, a large toroidal a transformer for high voltage. While the transformer department is well covered with oversized units, it may be placed to closely to sensitive signal paths. Alternatively, the grounding may be an issue. It doesn’t have that typical “ground loop” sound, but it still could be the issue. Either EMI or ground loop issue, the amp hums and it won't stop. Yes, it does get better with some powered tube time, but it's a sound you definitely don't want to hear in a unit of this price range.

Con 2: Wiring? The Chinese power tube in the first unit worked OK for the first hour and then the sound died, like the sound of a tube quitting – the volume was super low after that hour, almost immeasurable low. I chalked this up to a bad tube, but did not want to take chances and told myself to return it the next day. The next morning, I tried it out before returning it. The volume went completely back to normal. I suppose the tube issue somehow fixed itself, but not sure, so I listed for a good while longer. I put in about 8 hours of listening. The volume cut never became a problem again, but the hum I experienced with both units was there. So I returned the first unit and received the second unit today. The second unit introduced a few voltage spikes into my high end headphones. Again, I thought that the Chinese tube was at fault, so I dropped in a GE tube. The GE tube started off OK (still had a hum), but after about 30 seconds, the tube sparked like mad and almost destroyed my headphones! OK, so I did a double check with a Philips/Mullard tube and while that one did not spark up a storm and sounded fine, the hum was still present.
Conclusion: This unit has all of the potential in the world, but there are too many bugs for me to recommend it. It’s not ready for primetime. It’s also a risky to use – the second unit I received could have turned into a fire hazard if I wasn’t watching it closely. That said, if you end up with a unit that does work, it isn’t because of the quality control process, it was probably pure luck. If you ordered one of these, I wish you luck. My second unit is going back, and this time for a refund.