This is a review and detailed measurements of the brand new Monolith Balanced Headphone Amplifier using THX AAA 887 technology. It was kindly sent to me a couple of days ago prior to its official unveiling on September 10th. It costs US $399.99 and is slated for general release on October 27th.The Monolith is a direct attack on Massdrop THX AAA 789, aiming to better its performance while keeping feature set essentially the same. You can tell that from the overall design:As Avid readers of the forum know, there are now three headphone amplifiers that stand alone as the best of the best: the JDS Labs Atom , the Massdrop THX AAA 789 and Benchmark HPA4 . The Atom is great but is not balanced (input or output). The massdrop is but availability has been quite poor. The HPA4 is superb but very expensive. So having more options here is good and that is what we have.The controls are familiar to anyone who has seen the Massdrop THX AAA 789 . Three gain settings, jacks for XLR, 1/4 and 3.5 mm headphone jacks. And an input selector. The power button feels better than the one on Massdrop.The back panel is as you expect:As is the 24 volt "laptop" style external power supply.The Massdrop THX AAA 789 set a very high bar. Is it possible that Monoprice can exceed that? The number "887" may give you a clue as to them getting there. Don't go looking for that on THX website. It was an informal designation that was created just for this unit. To wit, official units will not have those numbers on it.The unit I received for testing is the limited production run. I am told it is representative of the retail products as manufacturing ramps up.Let's get into the measurements as that is what you are all here for.I must say I was not prepared for what unfolded in front of my eyes:This is stunning level of performance. Distortion levels are at or below -140 dB! SINAD is therefore determined by noise levels of the Monolith but also my analyzer. As it is, it matches the Benchmark HPA4 in that department:That is 2 dB better than Massdrop THX 789. From here on, I was crossing fingers that it would maintain this lead throughout the rest of the measurements.Signal to noise ratio at 4 volts input and output was exemplary at 123 dB:Dialing the output way down using the volume control to assess noise level for very sensitive IEMs we get better than 92 dB:Again, we have a 2 dB advantage over Massdrop.Let's look at intermodulation distortion relative to level across all three gain settings:A bit of a jungle in there despite me zooming in. Summarizing, the Monolith THX 887 maintains a few dB lead in noise and distortion department (solid lines).We can see that more clearly with THD+N versus power using 300 ohm:Neck and neck on power but slightly lower noise. Similar story exists at 33 ohm load:Switching to a50 ohm load we get:Here the performance gap is a bit larger. And with nearly 4 watts of power on tap, you should be able to drive just about any headphone.Frequency response is as flat as you can get:Channel balance shows some variations:Given the very low gain setting, I don't think in practice it will be an issue. The Benchmark HPA4 is the king of hill here if you want perfect channel match.Output impedance is comfortably low:My setup has close to a 1 ohm impedance of its own so the actual impedance is probably close to 0.4 ohm.I tested the Monolith THX Balanced THX Amp using my Sennheiser HD-650 using both 1/4 inch jack and "balanced" XLR. I had no need to go beyond middle gain setting. No matter how much I turned up the volume, there was zero hint of any kind of distortion. Fidelity was exceptional. With XLR connection and highest gain, you get scary loud so there is plenty of headroom here for higher impedance headphones.I then switched to Drop Mrspeakers Ether CX headphones. Despite the very low impedance of these headphones (around 25 ohm), there was no issues whatsoever driving them. Dynamics were excellent and nothing between your music and the headphones.I play the same tracks for headphone testing. Having heard them hundreds of times, I am sick of them so usually stop pretty quickly. Not this time. The fidelity was so good that I kept the amp running for the rest of the evening.There is not one subjective flaw I can identify with this amplifier.I did not think it would be possible to build a mid-priced headphone amplifier and beat the Massdrop THX AAA 789. It is clear that the Monolith team managed to go against the tide and produce even a better headphone amplifier. Both subjectively and objectively, the "Monolith by Monoprice THX AAA Balanced Headphone Amplifier featuring THX AAA 887 Technology" (yes that is the official name) beats the Massdrop offering. Mind you, there is not much room left to squeeze one up there but they did. It is like beating the world record holder in Olympics.Needless to say,It delighted me to see a company dedicated to absolute excellence and full transparency to our sources and content. This is a type of product you buy once and you are done. It made me happy just testing it!------------As always, questions, comments, recommendations, etc. are welcome.Thinking about needing to buy more test gear to handle the broader set of products we are testing. So please support us by donating using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/