Dan D'Agostino Momentum M400 MxV monoblock power amplifier

The Momentum M400 MxV Mono amplifier ($79,500/pair; footnote 1) is the latest iteration of Dan D'Agostino Master Audio Systems' debut amplifier of 2011, the Momentum Mono amplifier. Weighing 95lb, it is smaller and lighter than its entry-level sibling, the more powerful, 125lb Progression M550 Mono amplifier ($47,500/pair), and is veritably dwarfed by some other monoblocks, including the flagship D'Agostino Relentless Epic 1600 (570lb) and the Karan Acoustics POWERa mono (231lb), which I reviewed in May 2023. But if the M400 MxV's rock-solid look and feel and its exquisite aesthetics—a sleek amalgam of silver and copper fronted by a power meter that glows green and radiates Rolex quality—are any indication, a helluva lot is going on beneath its showy exterior.

Once I took a listen, I discovered sound so smooth and satisfying that the refrain "How ya gonna keep 'em down on the farm / After they've seen Paree," from a WWI-era song about soldiers not wanting to return to their family farms after experiencing big city life in Paris, kept playing on repeat in my head.

Direct from Cave Creek
What kind of amplifier is this that distracted me so from watering the cows and milking the chickens? D'Agostino's website (footnote 2) offered a few clues. The Momentum M400 MxV is hand built; it includes discrete, direct-coupled, balanced circuitry and boasts a "new, ultra-quiet 2200VA linear transformer" with a winding scheme that is said to reduce the hysteresis effect by 90%, minimizing power and thermal loss. The new winding method also plays a role in increasing current capability by more than 50% and "offer[ing] greater headroom during dynamic passages," which is surely aided by a bank of nearly 100,000µF of supply capacitance. All that adds up to potential output power of 400, 800, and 1600W into speaker loads of 8, 4, and 2 ohms, respectively.

For additional insight, I turned to the man himself, Dan D'Agostino, whose early designs (with Krell Industries, which he founded) redefined high-end sound. In a phone conversation spanning topics from audio to aging, we discussed the design principles behind the Momentum M400 MxV monoblock.

"The goal with the Momentum was to make something that looked very pleasing physically and had plenty of power to drive any speaker," Dan said. "The other goal was to make the Momentum sound as good as it possibly could. I didn't care about cost or labor; I threw in whatever technology was needed.

"The MxV upgrade is the biggest improvement I've made to the Momentum in the 12 years of its existence. The Relentless is clearly the best amplifier we have sonically, but it's also the most powerful. I love the Momentum chassis, but its size dictates how much power I can get out of it. At more than 400W into 8 ohms, we're pretty much at the peak. It's a very, very powerful amplifier in that package, and it's chock-full of technology from the Relentless."

According to D'Agostino's website, the Momentum M400 MxV's input stage has been "re-envisioned," with new circuit topology and new bipolar transistors said to deliver "ten times the current and power of the original devices." These components, the website claims, "extend low and high frequency performance with reduced distortion in both frequency extremes."

The M400 MxV's driver stage is completely different from the one in former Momentum models or the current Progressions. It implements the Relentless line's "inter-digitated style of driving output devices," as Dan put it, which uses multiple high-current drivers. "At idle, it's not doing much work," Dan explained, "but when it's playing music, it's able to deliver a lot more current."

A new bias stability circuit provides a nearly 50% increase in operating bias, the website informs—more class-A goodness without overheating, because the Momentum's stylish copper heatsinks keep things cool.

Innovations in the output stage have also trickled down to this Momentum from the Relentless line, including the output devices: 26 new, high-output transistors run at a fast 69MHz, "permitting the Momentum M400 MxV to achieve incredible bandwidth," the website states (footnote 3). The design is optimized for maximum thermal transfer from the transistors to the heatsinks. A simple tweak—a capacitor-resistor network connected to each transistor—ensures stability with low-impedance speakers. Indeed, D'Agostino claims the MxV can drive any speaker to its maximum output. "It's a technical marvel for what it can accomplish, given its size," he noted proudly. "There is no room left in the inside of that amplifier. You can't even put a pencil in there. Yet we've managed to isolate all the stages and components from each other and from outside noise.

"The Momentum takes a tremendous amount of time to build because everything must be located with painstaking precision. The Progression [which uses none of this trickle-down technology] gives me some room internally and some economies of scale for assembly. I can make probably two or three Progressions in the time it takes to make one Momentum. "If I didn't have my company, and the only audio product I could buy were one of my amplifiers, it would probably be the Momentum. It is not a budget product, but it gives you the most of our type of sound for the least amount of money."

Our type of sound? What's that? On prior occasions, Dan had described the Momentum's sound as more refined than the Progression's, but I wanted to know more. "We're trying to get as close to the musical event as possible," he explained. "I want to see into the music and hear who's playing the background. [That] is what I listen for in every product. I know the vocalist is going to sound great and the instrumentation will be spot on. But I want to hear the musicians in the background.

"I never thought it would be possible to hear them with clarity until I made the Relentless. I like a particular old Billie Holiday cut (footnote 4). She probably had some of the best back-up musicians that were available in her day (footnote 5), and I want to hear them. Lo and behold, with the Relentless I can hear some of the changes on their instruments. I can even hear the musicians breathing back there. That's the stuff that can give you goosebumps ... if you can hear it.

"After I designed the Relentless, I brought it home to see how much of its output stage we could incorporate in the Momentum M400 MxV. I laid out four different boards and tried them in a Momentum M400 MxV. I was just awestruck by the amount of depth and space created by the change. Then I realized I could accomplish even more by working with the input stage and the drivers. I did it and got very close to the Relentless sound. It doesn't do everything the Relentless does but, boy, when it comes to space and depth, it is right there."

D'Agostino and measurements
With every D'Agostino product I've reviewed—this is the fourth—some measured results have correlated poorly with what I was hearing (footnote 6). I asked Dan to comment.

"I also use measurement techniques," he replied. "I have the same equipment John has, and I have some other equipment that's probably more than Stereophile wants to buy. But I use measurements to see if I'm on the straight and narrow performance-wise—to make sure that the product doesn't do anything stupid.

"At Krell, I lived and died by measurements. Make no mistake about it: Here, I don't care about them. They're only a means to an end. If I make something that has higher distortion than the product just before it but sounds infinitely better, that makes me happy.

"In the end, I'm only interested in how good a component sounds. I'm never going to make something that sounds impolite, buzzes, hums, or only works with a certain speaker. But I do not care about the measurements when I look at my Audio Precision or HP stuff. When it shows me a reasonable output and indicates that it's doing a good job, that's all I care about."

To audition the M400 MxV in an optimal setup, I paired it with a D'Agostino Momentum HD preamplifier. These two components offer balanced inputs and outputs only, so obviously I used XLR interconnects for the pairing—alternately Nordost Odin 2 and AudioQuest Dragon. All the D'Agostino products I've reviewed so far come with hard rubber feet. To my ears, good after-market footers make an audible difference. Wilson Audio Pedestals answered the call.

For months, I've used the Stromtank S 2500 Quantum MK II battery power source to power my reference Progression M550 monoblocks and my entire front end. Thus powered, the M550s sound just right to me. Dan, however, has always maintained that his amplifiers sound best plugged directly into the wall. I've tried that; I've also plugged the M550s into the AudioQuest Niagara 5000 power conditioner. Invariably, the Niagara 5000 and the Stromtank resulted in a smoother, more musical sound than when the M550s were plugged directly into the wall.

However, I had recently upgraded my electrical service. As I detailed in my last review, I replaced all the aluminum wiring that the electrician could access with copper. Thanks to Ed DeVito of Audio-Ultra, I also replaced a breaker panel that used aluminum buses, and I've installed an Environmental Potentials EP2050EE surge protector/filter to absorb noise between 3kHz and 1MHz, generated by appliances and heaters (footnote 7). So, this time, when Dan advised that the M400 MxV is more "robust" with wall power and that the leading edges of low-frequency notes would sound better, I gave it another try.

For the first time since music filled my dedicated music room, I preferred the sound of an amplifier plugged directly into the wall. All the other components were powered by the Stromtank. It was time to listen.

Footnote 1: MxV stands for Mass times Velocity, which in physics equals momentum.

Footnote 2: See dandagostino.com/products/momentum-m400MxV-monoblock-amplifier.

Footnote 3: I asked what it meant for a transistor used in a class-AB amplifier to "run at 69MHz"; the answer is what you'd expect: They don't. It's a specification—these transistors are rated to switch that fast, which means that they should have no problem with very high frequencies, far beyond the audible range. Note that this isn't new: The transistors D'Agostino used in the original Momentum amplifier from 2011 were also said to run at 69MHz—see Michael Fremer's review.

Footnote 4: Billie's later version of "Good Morning, Heartache," released in 1946 on Lady Sings the Blues, first on Clef Records and then on Verve Records.

Footnote 5: The date was June 7, 1956. Charlie Shavers was on trumpet, Tony Scott on clarinet, Paul Quinichette on tenor sax, Wynton Kelly on piano, Kenny Burrell on guitar, Aaron Bell on bass, and Lenny McBrowne on drums—plus Billie Holiday of course.

Footnote 6: Such disconnects exist in many of our reviews, not just with D'Agostino.—Jim Austin

Footnote 7: For details, see my review of the Karan Acoustics POWERa mono amplifier.

Dan D'Agostino Master Audio Systems
5855 E Surrey Dr.
Cave Creek
AZ 85331
(480) 575-3069

jimtavegia's picture

I do like his comment about distortion as with so many liking tube amps this makes sense to me. It is also true as you review and then measure so not to be swayed by what JA1 finds out.

As hard as this is for me to say, this pair of amps at their price compared to some of the prices we read about at the recent shows that are 6 figures, this pair of amps may be a bargain for those who can afford them, as well as the fact that they look superb.

David Harper's picture

I promised myself some months ago that I wasn't going to troll this forum anymore. And, so far, I have kept this promise. 80K. 2 mono amps.
Bargain. God give me strength.

Anton's picture

Well, entry level integrated amps are now 22,000 dollars, so keeping our betters fed seems to be working!

noamgeller's picture

My youtube channel started to recommend me
Interviews of homeless people in LA and the surroundings...many of those poor souls are truly fascinating, illuminating even. Now stumbling on this article and the sentence contains "bargain" must means that something is really rotten with us humans. Shame, shame on us, truly ):

RobertSlavin's picture

I just want someone from the Dan D'Agostino company to explain why this is a reasonable price for this pair of amplifiers.

The reviewer should have asked the company this question. But Serinus never asks this basic question of the manufacturers of the many expensive products he reviews. Price is a very important consideration in a review of a product but not one he seems to pay attention to.

TJ's picture

Congrats JVS on the successful outcome of your AC rewiring project (!) and thank you for this fine review. As a long time Krell user, I’m always intrigued to see what Mr. D’A is doing next.

As compelling as the SQ of his latest amp must be, I can’t help but wonder how it would compare to an SPL s1200, or even the smaller s800 which I am listening to as I write this. These amps have an interesting gain stage with 120V rails, a minimal number of output transistors in a cascode configuration and exemplary build quality. Their surprising clarity, stage depth and deep bass are even more interesting. Definitely worth a listen if you have the opportunity.

MZKM's picture

The Rotel Michi M8 is more my aesthetic, is <1/5th the price, has less distortion and a ton more wattage.

I would like to see the people for who this aesthetic is for.

Anton's picture

This amp would look perfect in Admiral Harriman Nelson's quarters on the USOS Seaview!

Maybe even better in Captain Nemo's rec room on the Nautilus!

It's got more of a seapunk look than a steampunk, but that is just a matter of nuance!

georgehifi's picture

Doubt any Class-D could ever match this kind of BJT powerhouse.

Real amplification for those nasty hard to drive of speakers which only sound stunning when driven correctly.

Cheers George

Jason Victor Serinus's picture

As I prepare to start coverage of T.H.E. Show on Friday, I see multiple comments about the term "bargain" and scratch my head. This is not a term I used in my review. Nor is it a word that I would ever apply to monoblocks that cost almost $80,000/pair.

Products cost what they cost. Those who can pay the price enjoy them; those who cannot have alternatives. The Rotel Michi, mentioned above, is a product I also reviewed in stereo form and praised highly.

jason victor serinus

Ortofan's picture

... decade, DDMAS did offer an entry-level "bargain" stereo amplifier in their version of a plain, black box. The sole front panel adornment was a copper plate bearing the engraved signature of its creator. The Classic Stereo amp then sold for a mere $13,500.

That model is no longer in the current product line-up. The entry-level stereo amp is now the Progression S350, for about $27K. Apparently, DDMAS customers are unwilling to forgo the fancy chassis and the ostentatious power meter.

haroon's picture

John; kindly explain the distortion spikes that coincided with the zero-crossing points in the signal's waveform. Where is that, "more class-A goodness" in this new version?

John Atkinson's picture
haroon wrote:
John; kindly explain the distortion spikes that coincided with the zero-crossing points in the signal's waveform. Where is that, "more class-A goodness" in this new version?

Yes, the second-harmonic waveform is overlaid with spikes at the sinewave's zero-crossing points, but the level of the distortion is still very low, at 0.029%.

As the Momentum 400 MvX runs very hot, I suspect that the designer adjusted the output stage's bias current to ensure that the amplifier didn't overheat.

John Atkinson
Technical Editor, Stereophile

David Harper's picture

why is this forum so dead? Is it because most people have outgrown interest in audio gear? I'm asking a legitimate question. Or maybe young people just don't understand why anyone would care more about gear than about music? This hobby (if that's what it is) is dying. We are an anachronism. Not that I care. Change is inevitable. But it is an interesting question isn't it.

stereostereo's picture

All questions could be somewhat interesting based on their validity. Maybe you care about the gear more than the music but that has not been my experience based on my audio retail store. At my most recent Woofers and Whiskey event I had over 30 people here between the ages of 15-23. And more than 50 between the ages of 25-40. And the main driver has been overwhelmingly ones love of music. I stress to the industry folk who help me with these events to "just play music and have fun." No discussion of crossovers or woofer materials. And people have really responded and the discussions usually center around The Beatles, Frank Sinatra, Frank Ocean or Frank Zappa. That ain't about a hobby it's about a passion for music. And honestly I have not seen that much change in our industry. Sure the gear has gotten better, prettier, Hi res audio, etc but the one element that has remained constant is our passion for music. And that will never change.

Glotz's picture


Anton's picture

700,000 plus visits per month, dead or not dead?


On the other hand, Audio Science review gets 1.3 million plus.


Absolute Sound, 390,000


I guess the question is how 'lively' qualifies as not 'dead?'

Glotz's picture

And yet no response to your circulation figures as an "I'm wrong, sorry."

If he went to an audio show he would see that the places are jammed with tons of people with a variety of backgrounds and ages... and spending levels.

Time to come out of the house, dude. There's a whole world out there! And it's FUN!