ZMFheadphones Aegis Amplifier

Disclaimer & intro

It is always great to see Zach (Zach Merbach, the man behind ZMF), his wife Bevin and the wonderful ZMF team at CanJams. After reviewing the Caldera open, Caldera closed and Bokeh closed headphones, this time Zach kindly lent me the new ZMF Aegis amplifier for a few months in exchange for my honest opinion. I am not affiliated with ZMF, and soon will be returning the review unit, so everything you read here are the honest thoughts of a regular audio enthusiast. Warning: there is a smaller plot twist at the end.



What is the ZMF Aegis?

The Aegis is a tube amp with a unique, one of a kind circuit design. Currently there is nothing like this on the market. The brain behind the Aegis is Keenan McKnight, a.k.a. @L0rdGwyn. This amplifier started as a DIY project out of love. The road to the ZMF production unit we see today was long with unexpected turns. The Aegis is also available as a DIY project, Keenan wrote a step by step guide for ambitious amplifier builders. The guide is simple and takes you through the building process in a very clear way: anyone with basic soldering skills and experience should be able to follow. The parts will roughly cost $2000, so there is quite a bit of savings to make, as long as you are happy to invest time and effort from sourcing and buying parts to building.

For lazy and/or unskilled mortals like me, there is the ZMF version of the Aegis readily available for $3499 plus tax. The story of the commercial version goes like this: Zach heard Keenan's DIY Aegis back in 2022 and fell in love with the sound. Zach then built his own Aegis (which I had the fortune to try) and he decided this amp deserves a wider audience. To cut a long story short, he struck a deal with Cayin, a reputable Chinese amplifier (and DAP) manufacturer. Cayin builds popular and great amplifiers which all are great value at their respective price points. (IHA-6, IHA-8, HA-2A, HA-3A, HA-6A and the venerable HA-300MKII or the brand-new Soul 170HA.) It is safe to say that they know how to build good tube amps; the manufacturing of the commercial Aegis is in good hands.


Zach's first DIY Aegis. As far as I know, since then he built another one.

Special circuit

The Aegis is a SET (single ended triode) amplifier, but with a twist. In a way, the circuit is a mixture of OTC (output transformer coupled) and OTL (output transformerless) design; basically a transformer coupled amplifier wired as a transformerless amp. The output tube is wired as a 'cathode follower', which is different to how tubes are wired in a typical SET amplifier. Cathode followers are much more commonly used in OTL amplifiers, because they have low output impedance and low distortion. Without a transformer, a tube has to have low output impedance to drive headphones, so the Aegis is a cathode follower, but also pairs it with a transformer. The tube is wired as it would be in an OTL amp, but instead of being capacitor coupled, it is transformer coupled. This allows for removing a capacitor that normally would have to be used in a single ended triode amplifier. In the Aegis there are no signal capacitors at the output stage. Removing capacitors from a single path increases clarity, soundstage, realism which according to Keenan are the biggest strengths of his circuit.


The inside of the Aegis looks neat and organised: good job from Cayin.

A little recap for those, who are less familiar with these terms: OTL amplifiers typically pair very well with high impedance dynamic headphones, but using low impedance planar headphones is not recommended with them. Low impedance planar headphones often sound gooey on OTLs, if specs allow to plug them in at all. That is where the more versatile and generally cleaner, crisper sounding OTC tube amps come into the picture. Planar (and dynamic) headphones (usually) both sing on OTC amps. While OTLs require a more acquired taste for their warm and lush sound, OTC tube amps offer the fluidity and naturalness of tubes with a slightly more neutral tonality to a wider audience. What makes the sound of the Aegis distinctive and unique is the lush OTL tonal characteristics with the control and technicalities usually displayed by OTC tube amps.



Background and approach to this review

I am relatively new to tube amplifiers, only purchased my first real (non-hybrid) tube amp, the Cayin HA-3A, two years ago. During these two years I learnt a lot about tubes and tube rolling, and I believe I managed to max out the capabilities of the HA-3A with some rare and fabulous sounding tubes. (Raytheon 22DE4 rectifier, Mullard E80CC input and Bendix 5992 output tubes.) After hearing Zach's DIY Aegis at CanJam, I thought this could potentially be the next step for me, so I was more than happy when Zach offered the amp for loan in exchange for this review. When those stars align!
I also extensively auditioned some other tube amps multiple times, from the excellent sounding entry level Cayin HA-2A through the popular Feliks Envy to quite a few top of the line tube amplifiers like the Trafomatic Primavera, Viva Egoista 845, Headonia 300B. To me tube amplifiers offer a unique fluidity, a naturalness that solid state amplifiers simply cannot match. Contrary to the popular misbelief, tubes are not always warm and slow, they can be quite neutral and snappy too. The common denominator between all kinds of tube amps is this natural flow of the sound which most SS amps do not possess, unless you really go high up like the $15.000 Mass Kobo 465.


My Caldera with the famous Mass Kobo 465.

In this review I will share my thoughts on the Aegis as a HA-3A owner, which is hopefully helpful to folks who are considering an upgrade from tube (or SS) amps from the $1000-$2500 price category. I will reflect on some of the more expensive tube amps I mentioned above, but since I do not have them at home for direct comparison, I won't go into depth on that front.

Gear used

Eversolo DMP-A6 streaming Tidal/Qobuz (upsampling to 192kHz), Excalibur USB into Holo Cyan 2 R2R DAC (with purple fuse). Also tried HQ Player from PC playing FLAC, DSD. Chord Clearway interconnects to Aegis. On the headphone front I mostly used my beloved Caldera open with Grand Silver Palladium cable (great synergy!). Also tried most other ZMF headphones for short periods: Atrium open, closed, Verité open, Aeolus, Bokeh closed, Caldera closed. Plus, the Meze Elite briefly, but 95% of the time listening was done with my Caldera open.


Stock JJ tubes in the Aegis.

Build

Those kind people at Cayin know how to put an amp together: the Cayin built commercial Aegis feels solid. The wooden front plate makes the Aegis a wonderful visual match with any wooden cup headphones. Also, who does not like VU meters and tiny details like the ZMF logo on the meter plate?
There is no cage on the Aegis; admirers of tube beauty rejoice! I will come to it later, but there is no need for a cage on the Aegis, at least not against wi-fi and other radio wave interference. This amplifier is noise-free and interference-safe.
The colour of the transformer covers is not quite to my personal liking, but I can live with it. As a whole, it does match the aesthetics and vibe of the amp.
At some point in the future there is a good chance for different Aegis wooden front plates, perhaps the transformer covers will be customisable too. (This latter is just my wild guess.)
If I had to nitpick one thing, that would be the firmness of the tube sockets. They are not bad on the Aegis, all of them took my intense tube rolling with ease, but on the Cayin HA-3A the sockets do feel firmer.



Aegis is a heavy beauty; it weighs in at 21kg (46lbs). Good quality transformers are heavy; if your tube amp feels lightweight, be suspicious. Combined with a size of 36x29x20 centimetres (14x11x8 inches) the Aegis is not exactly desktop friendly. This amp demands and deserves a dedicated space in your room.

The back of the amplifier is simple: you have your single ended inputs, and that is it. There is no need for more on an excellent, single ended triode amp. On the front you have three headphone outputs, with variable output impedances. The three settings on the SE output are 5, 11 or 31 ohms, the XLR output's three options are 11, 28 or 85 ohms. The 4.4mm output is attenuated for use with sensitive IEMs: the selector is meant to set to 'low', providing 2 ohms resistance for IEMs rated above 105 dB sensitivity. (The DIY Aegis only has two output options: 5 and 17 ohms.)



The Aegis is capable of handling high input voltages, delivering an average of 2 watts output. This means Keenan's design circuit can drive the Susvara, the Tungsten but it is also quiet with sensitive IEMs. Not many amplifiers can do the same at this price point with such ease.

Let me take the opportunity to say a word about single ended and balanced designs. Balanced design (when left and right channels are completely separated) has become a marketing phrase these days. Many people think balanced is superior by nature, but this is not entirely true. A lot of cheaper amplifiers are not truly balanced to start with, and a lot of ultra-high-end amplifiers are actually single-ended. A balanced circuit's biggest advantage in most cases is more (double) output power, which often is actually quite unnecessary. The other advantage is a lower noise floor, which can be very tricky (expensive) to achieve in a single ended circuit. My point is, that we should not judge an amp just by reading 'balanced' or 'single ended' on the specs sheet. Circuitry designs are a lot more complex and complicated, and we cannot generally say that one is superior to the other.


GL KT66 power tubes: warm and lush.

Sound

General

If I had to summarise in a few words how the Aegis sounds, I would say it is dynamic, agile, energetic, clean, natural with just a hint of tonal warmth that gives voices the sort of lifelikeness only tubes can provide. The Aegis is the most dynamic sounding tube amp I have heard to date. We often associate solid state amps with clinical precision, speed, dynamic impact, clarity and micro details. Tube amps are often associated with warmth, mellowness, smoothness: a pleasant sound that lacks speed, ultimate clarity and micro-detail. Let me tell you now, the Aegis somehow has the best of both worlds: outstanding dynamics with speed and clear impact combined with the pleasant tonality and fluid naturalness of tubes.
These superb dynamics also come with an exceptional level of tonal density and note weight. Note weight is what makes sound real. Notes do not just float in the air providing an imaginary picture of the performers, but instruments and singers are firmly grounded with a solid foundation. This makes the listening experience feel more realistic in front of our inner eyes.
My experience with audio gear is that after a certain point you mostly pay your money towards this enhanced sense of realism. Amplifiers around $1000 are (usually) already very capable technically: there is good extension both to sub-bass and treble, there is depth not just width to the soundstage, there is good tonal balance. As you go higher and higher on the ladder of high-quality amps (and DACs or headphones) the most obvious improvement is usually an enhanced perception of lifelikeness. The Aegis with its speed, dynamics and note weight combined with the pleasant and natural tube tonality provides this sense of realism to a very high degree.


Pitch black background, where background details pop.

Bass on the Aegis is clean, punchy, and balanced. There is a great extension to sub-bass, which is often not the case with tube amps around and below this price point. Speed, weight and impact make bass heavy tracks super fun on the Aegis. There is nothing missing from the Aegis's bass that we often miss with tube amps versus solid states. Bass is articulated, present, but not overpowering. It is in balance with the rest of the sound: part of a coherent whole. The energy and agility bass tracks convey on the Aegis is ever so satisfying! (See track examples.) I would go as far as saying the Caldera on the Aegis is one of the most satisfying bass experiences one can have in this hobby: definition, clarity, separation, dynamics and impact are all sublime.

Mids are often the reason why tube amps are praised. In general, tube amps with their subtle warmth and fluidity lend naturalness and breathe life into vocals and acoustic instruments. These magical tube mids often come with some compromises: bass and treble roll off, lesser overall clarity versus solid state amps. There are no such compromises with the Aegis. The Aegis does have the sweet and lifelike tube mids we all desire, plus it is combined with clarity, definition and (keep repeating myself) excellent dynamics. On other triode tube amps from Cayin for example (HA-3A) or on much higher-end 300B amplifiers, mids can be a little smoother, a little mellower. Some listeners might find that presentation more appealing versus the dynamic nature of the Aegis.
On the HA-3A, when A/B testing the two amplifiers, this pleasant 'mid-fluidity' comes with some technical compromises. On the high-end 300B amps, mellower mids come with full technical competence, but also with a hefty ($12K+) price tag.
All in all, I find mids on the Aegis organic, natural and dynamic. At the same time this is one area where other tube amps may compete, depending on the style you like your mids served. Both approaches share the organic and natural base characteristics of tube sound, but one direction is mellower with a more liquid perception, while the other has more control and dynamics.

Treble on the Aegis is clean, clear, natural. It is not overly warm but has just a breath of warmth that makes the stage and performers come alive, sound real. There is enough air and plenty of speedy dynamics, good extension with zero fatigue. One of the many treats of the Aegis is that this exceptionally energetic and agile sound never becomes tiring. Sometimes I listened for 5-6 hours in one sitting, and I never experienced listening fatigue, just endless entertainment.

The Aegis holds one of the most important qualities of higher tier amplifiers, which is coherence. For this reason, it feels a bit artificially dividing to talk about bass, mids and treble as the sound does not feel divided but a complete, homogenic whole. Still, on paper, frequencies remain separable, so I did touch on them, but be assured: you receive a coherent picture.



Technicalities

A spatial 3D presentation is also a treat that is often associated with tube amps. How does the Aegis perform regarding imaging and spatial qualities? Excellent, as expected. Imaging is super sharp, pinpoint with accurate positioning and definition. There is enough air between instruments, but not too much. Width, depth and height is balanced, none of them is over-exaggerated. The Aegis projects a large, but not unnaturally large, balanced space. Thanks to the dead-quiet, black background, micro-details are very easy to pick up. Micro-details are not artificially exaggerated, as they can be on some brighter leaning solid state amps. All the details are there, but in a natural light. You can easily pick them up and follow them if you wish but will never feel like you are listening through a microscope. Micro-detail rendering remains excellent and naturally clean.

The Aegis's energy and agility never feels overwhelming; it remains exciting, entertaining and enticing. This does not mean that you cannot sit back and relax with the Aegis. If you like a softer and more laid-back sound, you might want to choose your headphones and DAC accordingly. For example, the Meze Elite with a softer leaning R2R DAC from Denafrips will definitely push the balance of your audio chain into more relaxed territories, even with the Aegis. On the other hand, if you like dynamics and punchiness while still having a natural and organic sound, the Caldera and a Holo Audio DAC could be your ticket. In my opinion, Aegis's main purpose is to entertain, and do that in a fatigueless way.

Another practical strength of the Aegis I have to mention is how easy and hassle-free it is to use different tubes. I rolled quite a few tubes in the Aegis in the past three months and never ever heard the slightest noise or hum. This is quite exceptional for a tube amp, and I am not sure what sort of design wizardry is responsible for this, but this quality certainly stood out for me. Also, I never ever experienced any interference or radio noise through the tubes: a tube amp where a Faraday cage is unnecessary. You are free to admire the beautiful tube glow 24/7 without hiding those beauties behind bars.

Tonality

It is untrue that tube amps always sound soft and warm. Tube amps can also sound neutral, even bright. For example, the Feliks Envy in stock form is more neutral than some solid state amps. Tubes can also be snappy. The Aegis has a healthy amount of neutrality, with just enough warmth to fill the voice of the singers and sound of acoustic instruments with life. Keenan's creation does all this in a lifelike, crisp and dynamic manner. Of course, both tonality and some of the technical aspects can be altered/improved by tube rolling. More about that soon.



Versus the Cayin HA-3A

I absolutely loved my HA-3A. It is a very good amp in stock form, that becomes a really great amp with tube rolling. The secret ingredient to make it sound great is using E80CC input tubes instead of 12AU7s. This is not officially recommended by Cayin, as theoretically it can slightly decrease the amp's lifespan by drawing more current than the 12AU7s. Cayin engineers 'unofficially' did confirm though, that E80CC tubes are within specs, so they are safe to use, Cayin just cannot officially add them to the compatibility list for the reasons above. E80CC tubes to my (and to other HA-3A owner's) experience are technically elevating the HA-3A quite significantly; most of all by improving dynamics. I combined my Mullard E80CCs with a Japanese Raytheon rectifier and Bendix 5992 power tubes. This combination for me was the best I rolled into my 3A and made me feel I managed to squeeze out the last drops of performance from this 'little' (13kg/28lbs) Cayin.



With this tube roll the 3A becomes a £2200 amp from the original RRP of £1500. Those 5992s are not cheap. The performance upgrade does show though, and the Cayin amp in this form in some ways comes quite close to the stock Aegis. In my opinion, with a tube amp we purchase a platform; a circuitry that is responsible for 2/3rd of the experience. The remaining third is the room where we can further improve the amp by tube rolling. The HA-3A is a nice amp in stock form, but the differences between the stock Cayin HA-3A and stock Aegis are just too apparent. The Aegis is technically a lot more capable. First of all, the Aegis projects a bigger soundscape with better depth. You can 'see' further in the stage; background information becomes a lot clearer and more present. The ambience, reflections are more palpable on the Aegis. There is more air between instruments, separation is superior.



Then there is tonality: the HA-3A is a lovely, mellow and smooth sounding amplifier with a focus on mids. Vocals are naturally flowing like some smooth liquid; it is extremely pleasant. The snappier and more dynamic sounding Aegis provides crisper vocals, and also better extension to both sub-bass and upper treble. These extreme frequencies on the Aegis sound more present and clearer, while the 3A's focus and definition runs out a bit.
If we compare the HA-3A with my magic tube roll to the stock Aegis, these technical differences become significantly smaller but still remain clearly present. When it comes to the choice between a maxed out 3A and a stock Aegis, I can see some people going for the 'E80CC/5992 3A' for that mellow mid sweetness, despite the Aegis still being a technical step up in dynamics, definition, background information, speed, impact and spatial qualities. Rolling tubes in the Aegis will further increase the gap between these amps.


Do not take this as an exact visualisation of performance difference, but more like a rough idea of what I am talking about.

Beside tonality, note weight is also a palpable difference between these amplifiers. The HA-3A feels a lot lighter, like it had less body to the notes. The Aegis has weight, it feels heavy, dense and real, but also quick at the same time. Note weight grants a feeling of realism to the sound: instruments feel more solid and three dimensional as opposed to the lighter and airier sound of the 3A. The background of the stage on the 3A is a little hazy while the clarity of the Aegis extends further into space. Drum hits are better articulated on the Aegis, they feel a little blunt in direct comparison on the 3A.
The Aegis, as most quality upgrades to my experience, simply conveys more information which enhances the level of realism. The 3A's strength is its mellow and sweet tonality. Aegis shows a higher degree of definition, with better articulated texture and timbre. On certain tracks, Aegis is able to keep busy passages clearer and better separated. Aegis's solid-state-like technicalities really shine through, especially in its bass performance. Clarity, dynamics and extension are no contest between these amps. These technical aspects improve a lot with tube rolling on the Cayin, but are never able to fully catch up.


Blue Bendix 5992 tube glow on the HA-3A with Mullard E80CCs.

Here I have to mention the exceptional synergy the Aegis has with ZMF headphones, especially with the Caldera (both open and closed). After my tube roll on the HA-3A, I really enjoyed the Caldera on it, even bass performance did not leave me unsatisfied. That said, I heard my Caldera on great solid state amps, like the Holo Bliss, or Mass Kobo 465 and realised that last bit of bass bite and dynamic impact is still missing on the Cayin.

What I gained with the HA-3A (lifelike, sweet mids, spaciousness) versus similarly priced solid state amplifiers like the Flux Mentor, Violectric V222, Ferrum Oor, HPA4 or Mjolnir 3, still made the choice for me obvious: I need tubes. With the Aegis there are no more compromises to make, even that last 10% of bass clarity and impact is there. The Aegis has such a grip and control on the Caldera transducers, that it feels like the snappiest solid state amp with all the benefits of a natural and fluid tube sound. Connect the Caldera with the ZMF Grand Silver Palladium cable and you truly have an exceptional example for the best synergy that can occur between headphones and amplifier.


Grand Silver Palladium cable: complements the Caldera in every possible way.

Versus the Feliks Envy and 300B big boys

I am not going into details regarding these comparisons, as I did not have these amps at home next to the Aegis. Still, I thought I would share some thoughts as I heard these amps more than once with my own Caldera. The Aegis in the USA is $3500 before tax. In the UK and in Europe £3400/€4000 ($4200) respectively, after paying our extortionate import taxes. The Feliks Envy starts at £6700/€7000, the Primavera is £13.000, the Egoista 845 is £16.500. In my opinion you have to climb the ladder this high, to hear a meaningful improvement. Diminishing returns hits hard at this level.

To my ears these top of the line tube amplifiers offer bigger space, a smoother and even more natural sound, superior coherence even compared to the Aegis. This is also true for the Envy to a lesser extent, but that amp sounded quite neutral to me: a bit more detailed but not necessarily more engaging. Of course, I know this could change with tube rolling, but the Envy did not blow my socks off at first listen. The Primavera and Egoista are definitely a step up from the Aegis, but at what price? You could buy an Aegis for all your family members and for your dog/cat from that kind of money. Those TOTL amplifiers are out of reach for most people. Even if they were within financial reach, you might want to spend that extra 10-13 grand elsewhere in life, instead of improving an already fantastic listening experience. In my opinion the Aegis represents exceptional value at its price point and is able to compete with some twice as expensive amplifiers, especially after some tube rolling.


Trafomatic Primavera (£13.000) and Viva Egoista 845 (£16.500)

It is a bit of a shame that I haven't had a chance to audition the Cayin HA-300MKII yet. That amplifier in the UK retails for £3800, so it is only a little more expensive than the Aegis (by 11%). The 300MKII is an amplifier that brings the 300B tube magic closer to mortal beings. At this point I am only able to offer a practical comparison point between these two, and that is the limited and much more expensive tube rolling on the Cayin 300MKII. That amp uses four rectifier tubes which can be a bit of a pain, and also the 300B tubes are limited to 4-5 options and are priced between $1000-$3000. Most people would not roll 300B tubes like we roll 6V6s, or even KT66s, KT88s. Despite the relative affordability of the 300MKII, for many potential customers this could be a strong point against it. The Aegis offers an unmatched variety of tubes one can roll and innumerable affordable specimens within those. If you buy a monkey (300B amp), you must have enough money for bananas (300B tubes) as they say. Luckily, Aegis is not a (very) hungry monkey.
Tube rolling the Aegis

The Aegis is already a fantastic amp in its stock form, but as with any tube amp I would strongly advise owners to dip into tube rolling, at least a little bit. If you are an experienced roller, the Aegis will be heaven for you. The number of possibilities is literally endless. This is the ever-expanding tube compatibility sheet for the Aegis, currently listing 20 rectifier tubes, 17 input tubes and 43 (!) different output tubes. Not all, but most of them are fully compatible with each other, and in some cases, you will need to use adapters. Still, the number of potential combinations is extreme and unheard of and could keep even the most vicious tube roller busy for years. Not everyone has to go that deep in the rabbit hole, trying 3-4 of the most popular NOS tubes could be enough for someone to find an even more pleasing sound that fits the taste better and leads to even more audio joy.

I like the Aegis in its stock form but was not completely blown away by the performance of the stock JJ tubes. They are dynamic, but the JJ E34 is a little two dimensional compared to some of the other tube types I tried. The stock sound is already nice and dynamic, but staging improves quite a bit with KT66, KT88, EL38 or KT90 power tubes. The stock rectifier and input tubes are good, but sound a little uninteresting compared to some of the NOS tubes I tried.


TAD KT88 power tubes.

TAD bundle
(5u4G / KT-88 / NOS 6N9S)

The first obvious choice for an inexperienced tube roller is the TAD bundle, that you can put in your shopping basket on ZMF's website when you purchase your Aegis. In my opinion with this good value bundle ZMF aimed for a more spacious and more neutral sound with better balance. The bigger space (depth) compared to the stock JJ setup mostly comes from the KT88 tubes which are 80% of the value in the package. The TAD bundle rectifier and input tubes while NOS, to me felt a bit too clean, too neutral, unengaging. If you generally like a more neutral and balanced sound, for the KT88s alone it is worth getting the bundle. They are definitely a step up in technical presentation compared to the stock JJs, but not necessarily the most desired tonality by tube rollers. To me the TAD KT88s proved to be a good 'neutral' reference pair to have around.

During my three months with the Aegis, I tried to check out as many tubes as I possibly could, but I only started to scratch the surface of what is possible. Beyond the stock and TAD bundle tubes I tried the following rectifiers: Chatham 5R4GY, Mullard GZ32, RCA 5R4GY (single getter), RCA 5R4GY (double D getter), Brimar 5R4GY, RCA 5R4GYB. The following input tubes: Tung Sol 6SL7GT VT229, RCA 5691 (red base) and the following power tubes: Gold Lion KT66, Ei KT90, Mullard EL38 (adapter required).

Let me share some very brief impressions on the tubes I have tried so far. From rectifiers I did not really care for the stock, the TAD bundle one or the Chatham. To me the RCA, Mullard and Brimar rectifiers were immediate and big improvements with a more pleasant and more natural tonality. The RCAs also increased dynamics, space and clarity. My favourite rectifier for now is the RCA 5R4GY with double D getters. This is not even expensive and brought me the biggest improvement so far.
Both the Tung Sol and RCA input tubes were again huge improvements on the stock or TAD bundle ones. These offer much better control, more lifelike and natural tonality. The RCA is cleaner, crisper and more dynamic, the Tung Sol has a fuller body and sounds a bit warmer, but with 10% lesser bass clarity.


Ei KT90 (type 2) power tubes, one of my favourites so far.

Regarding output tubes, as I already touched on it, the TAD KT88 sounds much more spacious and more neutral than the stock JJ EL34s. I believe, both in the EL34 family and in the KT88 family it is worth exploring further tubes. I am pretty sure, KT88 tonality can be improved with the sought after GEC version, and also the Mullard EL34 sounded significantly better to me on Zach's DIY Aegis versus the stock JJ EL34s.
The Gold Lion KT66 sounds warm with similar spaciousness to the KT88. Mids were sweet and engaging on the GL KT66 with big bass, although treble detail and sub-bass definition were lacking in comparison. This again, I am sure can be improved with better KT66 tubes, like the popular GEC.
The Ei KT90s (Type 2) are sought after NOS tubes and for a reason. These were a definite quality jump from the GL KT66 or TAD KT88 in resolution and overall refinement. A very nuanced, technical tube with pleasant tonality. These are not the most spacious sounding tubes, but the level of refinement they offer is more than enough compensation for that.
Beside the KT90s, the Mullard EL38s are my other favourite power tubes on the Aegis so far. These sound very spacious with excellent 3D imaging and layering. They are slightly less refined than the KT90s, but with a big and dynamic sound, excellent bass and pleasant tonality. As always, combination matters a lot with these tubes, and it takes time to find the best matches.

Tube rolling is a lot of fun, but I admit, it is not for everyone. Being happy and content with the stock tubes is absolutely possible and fine. Tube rolling can enhance the experience, improve the performance level of the amplifier or simply provide a different flavour. I would encourage my audio friends to try a handful of NOS tubes and see (hear) if they like what they find. I, for one, could not go back to the already fantastic stock tubes on the Aegis after 3 months of rolling. (You can follow my and other member's tube rolling journey in the Aegis thread. I have some ECC35 and EL39 tubes arriving soon.)

EL38 power tubes with anode cap adapters.

A handful from the hundreds of tracks I used for testing

Carbon Based Lifeforms - Nattväsen Great track in general, but also for background details, clarity and separation. The initial gentle percussion hits in the first 15 seconds reflect/echo nicely in the background, but you need a clean and clear amp to hear these echoes as separate notes. Also, the lyrics can be smeared on lesser amps, clearly audible and understandable on the Aegis.
H.U.V.A. Network - Something Heavens The track gets really busy after the 4-minute mark. Aegis keeps it nicely separated while the HA-3A for example cannot really keep up.
Heilung - Asja Staging and background information is clearer, richer and more articulate on the Aegis. Drum hits in the background have more definition, sound more real and lifelike.
Kurup - Joeira Lots of faint background hits and notes (especially the last 40 seconds of the track). These sound a lot clearer and more realistic on the Aegis.
Krumelur - Minimal Animal Great track for experiencing how hard and clean the Aegis can hit those bass notes.
DJ Shadow - Three Ralphs Experience sub-bass on a tube amp.
Sophie Zelmani - My Daughter Gorgeous vocal presentation. Weighty and clean on the Aegis, very lifelike. A little smoother and more liquid on the Cayin, but not as well-defined.
Massive Attack - Black Milk A classic. The background information, insight, clarity, depth and details will surprise many Massive Attack fans by discovering new layers of this masterpiece album.
Sync24 - A Deep Sea Meditation Experience Another masterful album from one half of Carbon Based Lifeforms. My most listened track on streaming and a go to test track for detail clarity. At the beginning of the track there is rain in the background, but on lesser amplifiers it can sound just like some background noise. On the Aegis (and Caldera open) the raindrops are clearly audible and certainly not just background noise.
Daemonia Nymphe - Deo's Erotas Realism and clear instrument separation is just sublime as the Aegis represents this track.
Frédéric Chopin - Nocturne in C-Sharp Minor "Lento con gran espressione", B49 from the For Wondering 2024 Chopin album.
This is a great recording and the Aegis with the Caldera open and Grand Silver Palladium cable offer the most lifelike, present and almost tangible piano notes I have ever heard in my listening room. I relatively rarely listen to classical music, but I had to restart this album once I finished it the first time and I keep coming back.

Conclusion

As you can probably guess after this rather long-winded review, I am quite fond of the Aegis already, and with newer and newer tube discoveries this love just keeps deepening. For those, who read through all the 6000 words, thank you; I appreciate your dedication and effort. Those who just cherry picked a few chapters, still thanks for landing here at the conclusion.
What can I say? The Aegis is a fantastic example of ingenuity, creativity, love, and passion. Thank you Keenan for creating this amp and offering the design to the community for free, with a clear guide to DIY build. What a rare and exceptional gift!
Thank you Zach for discovering Keenan's design and recognising the huge potential in it. Thank you to the whole ZMF team for pushing the concept of a commercial Aegis through until it manifested in what we see today. Thanks to Cayin for willing to cooperate!
In my opinion the Aegis is an exceptional amp, providing great value to performance ratio, even after European import taxes. This amp offers a unique and distinctive sound compared to everything else on the market. A dynamic sounding tube amp with solid state technicalities and tube tone/timbre: what a combination! A weighty yet speedy sound, creating a sense of realism that is usually familiar from much higher priced tube amps.
And the plot twist I mentioned in the intro? After living with the demo amplifier for three months, I decided that I need to purchase my own Aegis. What could be a bigger endorsement in a review, when the reviewer decides to go and buy his own unit?


Cheers, guys!

The first time I heard the Aegis amp was during CanJam NYC 2024. I brought some ZMF headphones from home and my first listening experience with it was in ZMF’s room. Once I started listening, the sound engaged me in a pretty compelling way. It was one of those magical listening moments that I’ve been chasing since I bought my first pair of high-end headphones. Most of the time though, as I’m trying things out, things may sound really good, but these moments of being so engaged and moved by the sound are rare. The Aegis was bringing that engagement with all of my headphones - a dynamic lifelike punch, an airy soundstage, and a beautiful midrange focus that gave me emotional engagement. It also sounded different from any other tube amp I had heard - there was just some things about its sound that I couldn’t quite place but I was also enjoying in an immense way. I listened to a house EDM track on the Atrium Closed and it sounded punchy and perfectly airy while keeping the sounds energized, a vocal oriented track I listened to on the Caldera was a perfect fit for its perfectly tuned midrange (Caldera offers my favorite midrange), and the Atrium’s romantic and lush qualities just seemed to jive so well with any song I put on. Oh man, this was good.

Zach had already offered to send me the Aegis sometime after CanJam so I knew I was going to get the chance to test it at home with all of my gear. And, as I own two really awesome sounding higher end tube amps - the Woo WA23 and the Feliks Envy, I was really looking forward to listening to them all against each other with my headphones. I was also considering selling my Envy at that point. I loved my WA23 since I first bought in in autumn 2022, but the Envy consistently left me feeling like its sound was almost there, but not quite, in an emotionally engaging kind of way. As soon as I heard the Aegis at CanJam, I thought it might be what I’d sell the Envy for to buy instead.

Many thanks to Zach Mehrbach for sending this out to me as well as Keenan for sending me some of his personal tubes. They made it clear there was no obligation to put out a review which I appreciated, but, here I am, wanting to share, because I find this very fun and hopefully some impressions in here will inspire and help others!






Once I got the Aegis at home, the engaged listening continued. It sounded like how I remembered at CanJam. The attributes I wrote about above were there and it was such a joy to listen to any headphone on. Indeed, it sounded different than my other tube amps: Envy, WA23 and the Woo WA8, a really good sounding little tube DAC/amp. I started rotating through my current favorite headphones while I listened on the Aegis: ZMF's Caldera, Atrium & Atrium Closed. It sounded utterly fantastic with everything. Its gain/impedance switch was helpful in matching with all of these headphones. Someone asked in the Aegis Head-Fi thread how Susvara sounded with it, so I tried that too. It was fantastic. It did not struggle at all to power it well and was a good synergistic match tonally. More on that headphone and a comparison with my other amps later though.



My chain at that point consisted of streaming from my Cayin N8ii DAP to either my Chord Qutest or Weiss DAC502 DAC into my choice of my amps and headphones. I was searching for a new source - something that could rip and hold my CD library as well as stream well. I landed on the Innuos ZENith. I was also looking for a new DAC that hit my preferences better. I liked the Chord and the Weiss for different reasons, but, having honed in more on my actual listening enjoyment preferences, I found that I liked ZMF’s sound the best. Some of the things I like best about their headphones is the realism, the midrange, a sense of space, lifelike dynamics and overall lifelike tone. Caldera especially hit all these marks in such a way that I’ve teared up listening to it many times at this point. It has become my favorite headphone. I wanted a DAC that could enhance these qualities as well as scale with the rest of my amps and headphones. I landed on the Holo May KTE.

Once I got the May connected to my amps, I could tell it was what I was looking for, at least in most ways. There was more space, a generally bigger kind of sound, more texture, especially in the midrange and everything sounded more real. The only thing I noticed that was different from the Qutest that I didn’t quite like as much was the bass: it seemed a little lean comparatively. More texture was there, but both the transients and the weight to the bass sounded just a bit leaner. Transients sounded a little softer. Weiss is known for its space, and the May sounded more realistic and also spacious, but ultimately less spacious than the Weiss and more realistic. So, it’s good to keep that in mind. DACs do influence the overall sound.

These earlier impressions (before the May and Innuos) were with stock tubes on Aegis. Keenan reached out and kindly sent me his favorite tube set: a pair of RCA 5691s, a pair of Tung-Sol 6550s and a Brimar 5R4GY rectifier. I continued to listen with these tubes, as well as did a little A/B with stock tubes that I write about later in this review. I also have WE300Bs in the Envy and Takatsuki 2a3s in the WA23. Everything else is stock in both amps (Performance Edition for the Envy). WE 300Bs offer more texture, a better midrange, and are a little warmer in the treble frequencies versus stock Performance Edition Full Music 300Bs on the Envy. Takatsuki 2a3s offer more detail and air as well as sound a little warmer overall on the WA23 versus the more midrange-focused Psvane 2a3s I had in before.

The goal was to improve my whole system with these things, but I didn’t expect it to make as big of an impact as it did. Aegis and WA23 benefited from the Innuos/May upgrade, but I was literally stunned by how much the Envy was transformed by them. So, most of the rest of the time listening was spent using the Innuos ZENith as my source, Holo May KTE as my DAC, and spending time listening to Aegis and then switching between it and other amps to compare. I’m also using Audio Art Cables Statement Series Cryo XLR and RCA interconnects, an Audio Art Cable HPX-1SE headphone cable with all ZMFs and Susvara’s stock cable when I used Susvara. I then switched back to the Qutest near the end to see how it sounded after months of listening with the May.



One of the first things I listened to once I got the Aegis at home was the Atrium Closed playing a house EDM album by Spencer Brown called Stream of Consciousness. I love the way this genre sounds on this headphone (especially my stabilized/resin set named Argemone). The sub-bass, warm tuning, dynamics, and especially sense of space (a result of the ADS - Atrium Damping System - I think Zach really figured out space with this - things sounds less headphone L/R like and more realistically dispersed) is so good that it’s just everything I want for this genre. Aegis played so well with it. This album on this headphone sounds great on any amp I’ve tried it on, but Aegis does enhance the AC’s qualities in a very engaging way and I was starting to recognize part of what makes Aegis so intoxicatingly engaging: the decay in the imaging sounds like it sticks around for a little bit longer than my other two amps, the WA23 and the Envy, even until the next transient hits. It does this in a way that doesn’t sound syrupy at all. The amp is still incredibly resolving and grippy at the same time. The second is that the midrange is so focused that lead instruments and vocals sound closer to you and intimate, also more-so than the other two amps. This happens while the soundstage is also big sounding. These things remind me of the Atrium. Vocals and instruments sound very close to you on the Atrium while the soundstage also sounds huge and spherical. And the decay lingers a little longer like the Aegis too. Naturally, Atrium sounds incredible on the Aegis. And it’s not too much of a good thing, either, where two of the same characteristics stack on top of each other to take away from having those positive things in a more measured amount. The bass is still well controlled, it’s punchy, and, if anything, the Aegis gives the midrange and the decay a little bit more so that it awakens these things on the Atrium in the best way. Anything I listened to on the Atrium or Atrium Closed sounded fantastic on the Aegis. No fatigue, just fun. I was switching genres, trying to see if there was something I wouldn’t like, but that didn’t happen and everything sounded new.



As good as those headphones are on this amp, the Caldera on the Aegis I think just may be even better. I spend a lot of time listening to this headphone on this amp and I think it comes down to a few things on why this is such a truly great pairing: Aegis’ midrange is focused in a way that feeds Caldera the texture and realism that it needs to come alive. That and the bass and dynamics come alive in a punchy, breathing-life-into kind of way that sounds unique. This is another quality, another thing about it that I hinted at before that sounded unique to me when I first heard it. It’s a sound that’s very visceral and ultimately words can’t quite capture the complete spirit of it, but I’ll try anyways: it’s like there’s a legitimate life-force energy that’s powering the Aegis to sound like it’s pumping air in a real, almost like you’re listening to live music kind of way. The WA23 and Envy are more powerful on paper (WA23 I believe is just above the Aegis at above 2 watts, while Envy is 8 watts), and do well in this dynamic department too, but there’s a quality to the Aegis that sets it apart from these that I find that gets my blood pumping. It’s like a little workout while I’m listening. I’d say headphones like Caldera and Susvara take the most advantage of this quality, but then, the more I listened, the AC and Atrium sounded even better dynamically because of it, too. All headphones benefit equally from this quality. That and there also seems to be a very strong specific synergy with Caldera.



People have been asking me how the Aegis compares to the WA23 and the Envy since I've had the Aegis at home, and I’m excited to share my impressions below.



Aegis and Woo WA23

Initial impressions while listening using the Caldera: WA23 more upper mids. Aegis is more warm in the midrange. WA23 has bigger soundstage and more air around instruments. Layers sound closer to you on Aegis. More clarity on WA23 and less of that fluid quality that Aegis has. A little bit more solid state sounding. Bass is more tight and more clear. Even more holographic and see-through. Punchy, but a little less so than the Aegis. Bass sounds more textured and more clean while Aegis’ bass sound a little rounder in comparison. WA23 sounds smooth, but Aegis sounds even more smooth.

One note on my listening experience here that I think is important to note: the WA23 has been my favorite amp since I got it in autumn of 2022. I’ve been comparing the WA23 to the Envy to the Aegis for the last few months and doing it so often that I wasn’t changing the gain and impedance switches on the WA23 much at all, both of which were set to high. When I was specifically changing between the Aegis and the WA23 for this part of the review, I kept finding the WA23 more forward and more bright sounding than I remember it sounding over the last couple of years. Had I just gotten used to the Aegis and Envy’s more warm sound? I then realized I had left the impedance and gain settings both to high. I played around with switching them down, realizing that high gain and low impedance is what suited Caldera the best. Dynamics were better, there was more room for sounds to breathe, things sounded less forced and forward and less bright. This applies not only to the WA23 - the Aegis can sound more this way with everything its gain switch to high (although not quite as bright and forced as WA23). This may be a good option for certain headphone or recordings, but it’s important to note that if the sound isn’t jiving, it’s not necessarily the character of the amp, but the settings.



Aegis and Feliks Envy

Initial impressions also using Caldera: Envy is more realistic sounding. Bass is more clean but sounds a little leaner (I remember bass sounded less lean on the Qutest on this amp) than the Aegis. The Aegis has more punch, but the Envy has more texture. More wide soundstage on the Envy. Bigger soundstage on the Envy. Elements are more separated out. Everything sounds a little closer on the Aegis. More cohesive sounding than the Envy. Better midrange on the Aegis. Vocals and main midrange-focused instruments like saxophones, trumpets, cellos, etc. are more centrally focused with the rest of the layers layered behind it. One of the things that makes the Aegis so engaging to me.



On jazz (Chris Potter’s The Dream Is the Dream). The Envy pulls ahead in soundstage depth and width, layering, resolution, and overall realism. It’s kind of insane how good it sounds. The Aegis sounds so good with jazz, too, in different ways: the saxophone sounds even more focused in the center, even more intimate. This makes the Aegis a little more engaging all around and especially more emotionally engaging. The imaging sounds more cohesive and less wide than the Envy, which helps with the intimacy and not feeling like you’re obviously wearing headphones. Lastly, reverb tails flow into the next transient in the most beautiful way, providing a beautiful, intoxicating atmosphere that’s perfect for jazz.



Aegis / Envy / WA23 back-to-back listening

My reasons for potentially selling the Envy was that it had a few things missing for me: an emotive, intimate midrange and sense of cohesiveness in the soundstage. It also had almost no play in the volume knob. It’s a sound in many ways that reminded me of when I owned the LCD-4: effortless and holographic sounding bass, incredible layer separation, incredibly detailed while also still remaining warm, and yet…. pretty V shaped (meaning the midrange was not as prominent). Envy doesn’t sound V shaped in the sense that the mids are dipped out, but midrange instruments image further away from you while the bass and airiness get centerstage, and I found that voices and main instruments were not as emotionally engaging as I felt they needed to be for my preference. I think I’m a more emotional listener and need that. Overall, I found the Envy to be an impressive sound, but after listening for like 5-10 minutes, I missed the midrange from my WA-23 and almost always switched back over. Unless it was jazz. The weighted sound of the Envy was one I preferred for jazz.

But then I got the May and Innuos. I was very surprised by how it transformed this amp for me. The sense of realism was so good that the issues I had with it before were transformed into sounding more like an aged musical instrument being played by someone who’s been playing well for decades - the sound came together as one. The midrange sounded better; more textured and holographic and ultimately more engaging. Voices were stunningly detailed - utterly real sounding and now more intimate and centered, which I found totally addicting. All of that and the other attributes of the holographic bass and effortlessness of sound being fed right from the May/Innuos source seemed to shoot the Envy into the stratosphere. The May/Innuos positively affected the Aegis and WA23 too, but they sounded great and held my attention before with my Qutest/Cayin N8ii source combo. This level up in sound inspired me to get some WE 300B tubes for the Envy, and, as I wrote earlier, that’s what I was using doing these impressions.



Sara Bareilles - “Islands”. Starting on Envy with Caldera. Piano sounds real. Bass is stunning. Voice is close and so much texture with it. Things sound very wide. Layers seems very well separated out. A lot of airiness in a great, atmospheric kind of way.

Continuing with Aegis: Layers seem more cohesive, but a little congested compared to Envy in the chorus. Vocals sound closer and every other layer a little behind it. I like this. So engaging. Bass doesn’t hit as hard (interesting, as my past impressions for some tracks have been the opposite) and doesn’t sound like it goes quite as low. Back to Envy. Yup, all the same impressions apply. Interesting note here: Envy is so revealing in the texture and air around her voice that it's more obvious she's in a vocal booth and it's not a live performance.

On to WA23. Midrange sounds more focused than Envy. Less wide soundstage. Bass sounds stunning in a similar way (using RCA, interesting to note) to Envy, but slightly less. Bass seems better controlled and almost separately controlled to other layers. Not as good as Envy though. Sounds like a cross between Envy’s big-ness in the bass and bigger soundstage and Aegis’ midrange focus and more focused soundstage. More texture and more detail in Sara’s voice too compared to Aegis.

Things sound the most real on the Envy. Even though there’s a weight to the sound, it sounds the least punchy out of all three. There seems to be a bit of an extra visceral wallop on the Aegis and WA23.



Joe Satriani’s “Goodbye Supernova”. Starting on Envy with Caldera. WOW. The bass sounds so holographic, the imaging is huge, everything sounds so lifelike and visceral. It’s unreal. A moment where the Envy and Holy May pairing really shoots into the stratosphere. Switching to Aegis. Everything sounds more cohesive. Still big, but less so. Layers are closer. More midrange focused in the guitar and Joe’s lead guitar sounds closer and tonally a little different. More intense. Envy much more textured though. A little more round and punchy down low on Aegis, but less holographic in the bass. Switching to WA-23. Sounds like the split difference between Envy and Aegis: everything sounds bigger again, but more midrange focused than Envy. I highly recommend giving this track a listen. One of the most impressive and mind-blowing tracks of this whole experiment.

Piano: (George Winston’s Night album and Brad Mehldau’s “Bittersweet Symphony/Waterloo Sunset”). More atmosphere and air around harmonics on the Envy. More brighter overtones on WA23 and Aegis. Most realistic piano sound coming from Envy. Most wide on Envy.

After listening like this back-to-back, since the WA23 is tonally more similar to the Aegis than the Envy, that it’s like taking Aegis and then upgrading it to an even higher flagship level. Which is saying a lot, as the Aegis is quite the flagship and technically capable amp in its own right.

Because the Envy was so transformed for me, I decided to keep the Envy and send back the Aegis. But were it not for that curveball, I would have bought it.

Summarized impressions:

Envy has the most realism. The most holographic, reach-out-and-grab what you’re hearing sound. It’s also got the most wide soundstage. It’s also got that specific “weight” to it that is unique out of all three. It’s got the most power at 8 watts, and you can feel that in the bass punch and open soundstage. That also translates to the highest noise floor of the three. Lastly, it responded the most to upgrades in the signal chain. It seems to especially love R2R DACs.

The WA23 has the most depth in the soundstage. It highlights the midrange and has the most upper mids versus the other two. It also sounds open and airy, even more so than the Aegis, but less so than the Envy. It sounds great in a very high-end, solid-state technical meets beautiful tube midrange kind of way. The Envy sounded the biggest - in soundstage and in the bass. The bass was tight and controlled and punchy, but less so than the Aegis. Aegis sounds a tad bloomier in comparison, less so with Keenan’s tubes. Aegis was more punchy but also a little less textured versus the other two amps. Envy was the most textured everywhere.

Aegis had the best dynamics, was the most exciting and had the most focused midrange. It had that liquid decay quality leading into transients yet stayed technical in its handling of transients. Midrange instruments sound the closest too, as do all other layers, even though the soundstage depth is really good. It’s the most exciting and in-your-face sounding of the three. No hum or noise either. Sounds like a black background solid state amp kind of way compared to the other two.


Some overall notes:

WA23 and Envy both benefit from Holo May KTE’s balanced output. Aegis has just one single ended input. The single-ended RCA output from May sounds great too, but the balanced output does sound better. I notice it most in the bass. The bass is even more holographic and actually sounds like it may even go lower and punch a little harder (may be the 5.8v output versus the single-ended 2.9v output there). Using single-ended output into both the WA23 and Envy to even the playing field a little more, all the above differences between these two amps and Aegis are there, but the bass I wouldn’t say is “looser”, but it’s not quite as utterly holographic and visceral as it sounds from the XLR. A very slight lessening in detail and air, perhaps? Very subtle if so. I may have been looking for differences where there weren't any. Bass is the defining obvious difference though.


Something that is interesting to note here is how long they take to hit their stride: Each take about 20-30 minutes to sound less stiff and sounding properly warmed up, but I noticed after about an hour and a half to two hours, the WA23 and Envy pulled into a category of sounding even better. The dynamics especially sounded even better at that point, near that life-force thing I was talking about with Aegis, but in their own way - more open, effortless and utterly into “wow” territory. Aegis sounded even better indeed at that same hour and a half to two hour mark, but the Envy and WA23 started to really pull even more ahead of the Aegis into a different category: I found myself mouthing “Oh. My. Gahhhhhd.” while I listened to either amp at this point. The sound leveled any headphone playing through up to sounding so good in a way that I think most audiophiles are hoping for when they get new gear or audition something at CanJam: things sounding so effortless, lifelike and open that it doesn’t sound like you’re listening to headphone anymore. You’re utterly transported to a different realm. The amps warming up for this long seems to be an important part of the equation. I didn’t notice a downgrade in sound anytime afterward, either, and my listening maxed out at about 5 hours while taking some breaks here and there.



Aegis and Woo WA8

While some may think comparing the $1900 WA8 to the $3500 Aegis may seem a little silly, I don’t. I think for most of us, it’s really about what specific sound you want and sometimes the least expensive option can have what you’re looking for. I’ve been very fond of the WA8’s sound since I got it and for me it hits the mark for my preferences. It has some special characteristics that make it very enjoyable to listen to. The DAC inside the unit is good and works well with it, but I use the Hugo2 as DAC when I listen. The pairing is synergistic magic. The resolving Chord sound with the WA8’s tube sound is an addictive listen. I use it as my night time listening setup, often using ZMF headphones and gravitate towards Atrium with it the most. It also powers planar and dynamics equally well, but is not as good of a fit for Susvara as Aegis is.



Spencer Brown’s “LA ID” on Caldera: Aegis is more tight and controlled in the bass compared to the WA8. Even more punchy, but only slightly. Layers sound a little closer than WA8, and yet, because of the longer decay, the Aegis still sounds even more airy and has the bigger soundstage. That longer decay is there and holds on even longer than on WA8. There’s actually that life-force kind of pulse happening on the WA8 that makes it magical like the Aegis, but in a different way. More in a warmer way rather than a more crystalline clear kind of way like the Aegis. The dynamics are also very strong, but Aegis is better. This sounds unique compared to the others, much like the Aegis, but in a different way. I suppose it can be argued that most tube amps have this kind of breathing dynamic quality that I’m talking about, but Aegis and WA8 do indeed do this in unique ways compared to everything I’ve heard until this point. WA8 seems to do it in way that sounds tonally warmer than the Aegis. The Aegis’ dynamics are more clear, clean and more resolving sounding than the WA8. I do notice more texture in instruments on the WA8 though, interestingly.

WA8 definitely warmer - vocals sound a little closer on “Chance On Us” in the lower mids. Not in a muddy way. Vocals sound more emphasized in the treble on Aegis.



Susvara on Aegis

I listened to Susvara on Aegis for a while to get a feel of the pairing. I listened to some different genres: singer/songwriter folk, jazz, and pop rock. Like so much that gets said about this headphone, my experience has been the same: I’ve struggled to get the chain right to enjoy listening to it at times. It needs more power to come alive as far as headroom, bass and dynamics compared to most other headphones. Aegis did not struggle in these areas. Aegis’ qualities that I wrote about before were still present while listening to Susvara. But, the listening experience reminded me of when I owned and listened to Susvara on the Ferrum Oor (with Hypsos power supply): the overall experience was pleasant, but left me wanting more. I don’t have the Oor anymore, and I do think Aegis is better than Oor in that dynamic quality and also from what I remember in soundstage, and overall has a more engaging and musical tonality. I was using the middle gain/impedance setting at first. I switched to high and, while it helped in some areas, everything sounded more compressed towards me, so I went back to the medium setting. Things sounded more accurately represented in space that way.

Switching to Envy gave me more (on medium gain/impedance setting too). There was more headroom, the bass deepened and sounded more full, and the dynamics opened up and sounded like it had more breathing room to display the layers. There was also much more texture and more detail. Aegis doesn’t lack there, but the difference was a little bit of a shock at first. One area though, and this is consistent with what I’ve written before, is that the midrange still sounded more focused, more intimate, and even more engaging while listening on the Aegis.

Switching to WA23 (high gain/lower impedance), those areas of headroom, bass, dynamics took a downward shift, but only by a little bit. The midrange was better and more focused, the soundstage less wide but sounded more deep. There was less texture with everything and the bass was a little more thumpy. Dynamics were great though. Envy still had more of an effortless quality there. Perhaps because the layers had more room between them to be heard.



Marcin Wasilewski Trio with Joakim Milder's Spark of Life album. Starting on Envy. So good. Weight to everything. Atmosphere is amazing. Detail is amazing. Switching to WA23. Cymbal hits stick out more. More cohesive soundstage. A little more upper mid and treble sounding. Saxophone sounds more focused and even more engaging. Less texture than Envy though. Things sound real, but not as real as on Envy.

Switching to Aegis. Things sound more cohesive. That decay is more present. Cymbals stick out, but sound more like a warmer splash than a more clear “ting” of the WA23. Saxophone still is focused, but sounds a little warmer than on WA23. WA23 has more detail and texture there.

Switching now to Caldera on WA23 with same album. Ah, I'm home.

What I find really interesting about Susvara is that, even though I’ve owned it for a few years, I don’t want to listen to it very often. It’s offered some of the most stunning listening experiences and yet, because it’s so revealing, minor annoyances become bigger, irritating annoyances. If the track is played well, mixed well and mastered well, it’s offered some of the best listening experiences I’ve had. It reminds me of hearing the Sennheiser HE-1 or the HiFiMan ShangriLa Jr. But when it comes to me being pulled to listen to something, it’s going to be a ZMF headphone most of the time.

It comes alive even more on the Envy. The bass, soundstage and detail are the best out of all three amps. Because of how revealing of a headphone it is, it can sound on the unimpressive and non-exciting side on amps that don’t quite give it the juice it really needs to come alive.

How this relates to the Aegis is that if I'm going to do Susvara - I'm gonna do it on the most powerful amp I have.



Caldera on Aegis with Keenan’s tubes:

Airy. The decay is amazing. Bass sound punchy but controlled. A warm sound, even though there’s plenty of treble energy and detail. Listening to “Message in a Bottle” by Marcin Wasilewski Trio.

Stock tubes (with upgraded "standard upgrade 6SL7" tubes Zach sent from Tube Amp Doctor):

Soundstage sounds a little bit less big and airy. A tad less decay than Keenan’s tubes. A little less detail. A little less punchy and also less solid state sounding. Bass sounds about the same? Less treble. More warm. Some of these things are subtle. Qualities of that unique decay and dynamic life-force punch are there, but less noticeable.

Listening again later. Now the tubes seem brighter than Keenan’s tubes. Interesting. Punchiness and decay not quite as good as before for sure, although still good. Keenan’s tubes definitely enhance these qualities. The bass sound a little more full but a tad less tight and textured.

Using the Chord Qutest again:

Qutest bass sounds a bit more punchy and also more emphasized versus May. Transients seem better overall. Cymbals and high frequencies stand out more on Qutest. Layers sound closer to you. Less air around instruments on Qutest. Transients seems a tiny bit snappier. Less decay. This is all on Keenan’s tubes.

Listening again later. A/B between Qutest and May. Everything sounds closer on Qutest, so I think that’s part of why the bass sounds even more punchy and in your face. Transients seems more intense than on May. Less air around everything though. Less space. Sounds more exciting though overall.


....in summary:

The most striking thing about Aegis is its uniquely addictive sonic stamp in having that life-force like dynamism and an intoxicatingly liquid decay. It holds its own with bigger, more expensive amps by sounding technically proficient yet wonderfully tubey in all the best ways. Plus, it has next to no noise floor, has VU meters, looks awesome and is a fantastic value. I think anyone who buys this amplifier will not be disappointed!

INTRODUCTION
I've heard big transformer-coupled tube headphone amps at shows and a few times in my system, including amps by Feliks, DNA, Woo, Cayin, Auris and others. When the ambient noise was low I often heard things I associate only with large, powerful tube amps: a deep soundstage; a big, expressive midrange; hyper-real spatial placement of notes and voices, each seeming to sit on its own little puffy cloud in 3D space; and sometimes a slight roll off in the upper treble and a somewhat less deep and dynamic bass. I assumed this is what all big tube amps do.

Then I heard the ZMF Aegis, which upended all assumptions and exceeded all expectations. It sounds different and is different. It's also gorgeous to look at.


The Aegis Backstory
L0rdGwyn (Keenan McKnight) is well known here for his innovative DIY tube amps, the OTL Airmid and transformer-coupled Aegis. A lengthy Head-Fi thread is devoted to them. He also supports these amps with very helpful, informative instructions and cogent answers to user questions. ZMF Headphones' owner/chief designer, Zach Mehrbach, is also well known here for designing a succession of beautiful (and beautiful sounding) artisanal headphones starting in 2014. Keenan and ZMF are collaborating on the commercial version of the Aegis. The ZMF Aegis is designed by Keenan, manufactured by Cayin, with design aesthetics and marketing by ZMF.

Amp Design & Tubes
The design of the Aegis is quite unlike that of other tube headphone amps. Keenan believes the Aegis is the first commercial tube amp to use this design. As Keenan told me,

The other point I would make about design is how much user-customization of output impedance the Aegis offers. There are 3 user-selectable output types (XLR; ¼"; and 4.4mm; switch on front panel). The 4.4mm is significantly attenuated for IEMs, so I'll focus on XLR and ¼". The Aegis is single ended; its input is an RCA L/R pair. The XLR is a pseudo-balanced output with the output transformer acting as a phase splitter; it's offered for user convenience. The XLR output puts out somewhat higher power than the 1/4"; it also has a higher output impedance at each setting of the impedance switch. Thus users can co-vary the output type with the 3 impedance switch settings to precisely dial in the output impedance that reaches the headphone. This matters because output impedance subtly affects the sound of the amp

The Aegis uses 5 tubes. On my loaner the stock signal tubes are 6SL7; power tubes are EL34, both JJ; and the 5AR4 rectifier is Sovtek. There are many possibilities for tube-rolling with this amplifier:


Conservatively rated, the amp's power output is 2 watts per channel. That is BIG tube power, plenty for anything out there.

Physical Stuff
The Aegis is a full-size tube amplifier. It measures 11½ x 14 inches and is heavy. It's also gorgeous to look at (apologies for the crappy flash pictures):



My loaner has verdigris-painted transformer covers, a teak faceplate, wooden volume pot & on/off button (a 1st in my experience) and a painted metal bar with enclosed ZMF logo running along the bottom of the faceplate. This mix of wood and earth tones is very attractive. The Aegis stands out. This design gets small details right: the figured circular brass rings encircling the base of each tube contrast nicely with the dark grey case, highlighting each tube from below. The Aegis puts on a discrete, attractive light show, with soft yellow light emanating from each of the two VU meters; the on/off button subtly outlined in orange light; and the volume pot circled by 16 small, orange back lights. And of course there's the soft orange glow of the 5 tubes. IMO this Aegis is knock-out pretty.



The Sound
After a day's burn-in I began listening via the ZMF Ori to check how much power the amp had for this relatively power-hungry planar (hint: a lot), then switched to my best planar and dynamic headphones, the Caldera open and Verite open. From minute-1, the sound characteristic that jumped out at me was clarity. I've never heard this much resolution from a tube amp. Even 50 year old studio tracks that were never conceived as "audiophile" recordings sound bigger, better, and more expressive on the Aegis. Every musical note rings clear as a bell. Part of this clarity is the truly black background; the Aegis is quiet as the proverbial grave.

Years of audio experience conditioned me to distrust apparently high resolution. Too often it comes at the cost of brightness, edgy transients, and spikes in the upper mids or treble. But the Aegis sounds nothing like that. This amp captures the soul of music. Its considerable resolution is natural, organic and dynamic. The amp's overall tone is neutral, well balanced and level, top to bottom. For me, the "voicing" of the Aegis is ideal—and this is with stock tubes.

I quickly became accustomed to Aegis-level resolution. I heard new things in tracks I've heard countless times: keyboard parts I could barely make out before; precise articulation of strings and percussion; ambience of the recording space. Latin Jazz tracks with multiple percussionists sound utterly clear; every drum strike is distinct. The vibraphone, so challenging to record, is beautifully and realistically rendered here. The Aegis' resolution extends to the accurate timbre and body of instruments and voices. With this amp, resolution enhances tone.

It's easy to get used to high resolution that is so musical. Tracks from different recordings sound noticeably different. With the Aegis I easily hear into any track and understand the effects the mixing engineer was going for. It's also easy to hear the slightest reverb or delay in microphone processing, the relative "wetness" or "dryness" of the recording acoustic, and so on. I tried a number of music genres with the Aegis, everything from ethereal classical to electronic pop with strong beats and bass pulse. The Aegis nailed every genre. This is not an amp that's good with certain things but not others. If there's a music genre that's "eh" on the Aegis, I couldn't find it.

Soundstage: I've never heard soundstage this wide and tall. At times it felt like a sonic bubble surrounding my face and head. Some sounds were actually behind my head. All 3 headphones I heard on the Aegis sounded even larger & more spatially expressive than usual. This amp's dramatic soundstage was heightened by the clarity of each note, precisely anchored in sonic space. The Aegis' soundstage could be huge, intimate, or somewhere in the middle; it reflects the music source. Tube amps often have strong spatial and soundstage qualities; the Aegis simply has more. When I paired the Aegis with the Caldera open, a headphone that has ZMF's ADS system and soundstages heroically, it was like sitting midcourt at Wimbledon.

• The Aegis' soundstaging can be startling. In a Scary Pockets live-in-the-studio YouTube video, the song's chorus features a loud, heavily distorted bass guitar. Those bass notes appeared suddenly beneath my chin and across the bottom of the entire soundstage. It felt like one those lead radiation aprons used for X-rays was draped around my shoulders.

Tone: Frequency response of the Aegis is easy to discuss because bass, midrange and treble are so equitably reproduced. I can hear no bloat or significant frequency elevations/depressions. Every instrument and voice sounds just right. Timbre is accurate and life-like. The bass deserves special mention. The Aegis has bass for days (including sub-bass), deep, textured and dynamic in ways tubes often are not. Treble is equally impressive. If there's a roll-off in the upper treble, I don't hear it.

• I'm reminded of 2 measurements (rise time [small signals] & slew rate[(large signals, measuring an amp's response time) commonly used for solid state amps but not tubes.
• I could be wrong (please weigh in, Keenan), but I'd guess the Aegis has a fast rise time and/or slew rate, maybe also a better than usual S/N ratio. This amp goes from 0 to 100 in no time.

At times I heard a slight forwardness of certain voices or instruments in the upper midrange of the Aegis. This was not present in all tracks, just some. I suspect what I'm really hearing is the mixing engineer's decision to highlight the lead vocal or instrumental. Either way, the effect is small. It's the kind of thing tube rolling could readily change.

Impedance Matching: When first listening to this amp I used the XLR output (all my cables are balanced). After Keenan explained how the XLR and ¼" outputs differ, I tried the ¼" output. Then the sound really snapped into focus:

• The ¼" output sounds more controlled and somewhat smoother than XLR, at least to me. I did most of my listening on this output
• I appreciated the ¼" output's somewhat lower gain. The Aegis is so powerful that the extra gain with the XLR output is not necessary, at least in my 2 systems
• The ¼" output has lower output impedance at all 3 settings of the impedance switch vs XLR. For me, "Low" worked perfectly with the two planars, and either "Low" or "Med" worked well with the VO. I didn't care for the "High" setting's higher gain, probably because both my systems have single-ended DACs (MHDT Labs Orchid; Metrum Onyx) that output well over the 2.0v standard

To my ears the lower impedance settings produced a somewhat smoother, calmer, more controlled sound. Perhaps I traded off a bit of dynamic punch for this (not sure). If so, the tradeoff was worth it (this amp has dynamics & punch to spare). The Aegis gives users unusually granular customization of sound via combinations of output type & impedance settings. This is both elegant & practical.



Tube-Rolling
The stock tubes sound very good. But of course, tube rolling beckoned. I replaced the stock EL34s with matched pair of NOS Siemens EL34s. After 5-6 hours burn-in I did some listening. The Siemens sounded subtly different, a bit sharper on transients and slightly tipped up; drums & percussion, already crystal clear, became borderline spectacular. But on less than ideal recordings, these tubes let me know it. Still, vocals stand out and the treble is sky high. Bass is very clear and controlled. This is pretty much what I expect from Siemens tubes—relatively small but audible differences.

Keenan believes the unique circuit design of the Aegis predominates in determining the amp's sonics. He also says using different types of power tubes in place of stock (ie, KT88s or any other compatible tubes) may produce more audible results. I suspect he's right ... but that doesn't stop me from wishing I had a pair of late '50s Mullard Blackburn EL34s to try in the Aegis.

Aegis vs Icon Audio HP8
It seemed unfair to pit a compact 3-tube amp against a 5-tube, full-frame amp, but the contest was far from a blow-out. Both amps sound rather amazing via the VO and Caldera Open. Sonic differences are small but meaningful:

• The black background of the Aegis continues to impress, but the HP8 comes close. Neither amp telegraphs "tube sound" -- there's no grunge or sonic dirt with either. Both are very quiet even with no music playing and volume pots at medium loud (impressive)
• Soundstage on the Aegis is somewhat wider and taller than on the HP8
• But soundstage on the HP8 is somewhat deeper
• Bass on the Aegis is articulate and textured. It's the best bass I ever heard from a tube amp, maybe the best bass of any amp
• But the HP8's bass was a bit more impactful and a touch deeper, albeit not as textured as the Aegis. This is probably due to the NOS RCA 6NS7s I use on the HP8. When I switched to them, the HP8's bass became noticeably richer
• The Aegis is fast and neutral; its articulation of multi-track music & ability to untangle complex passages is a next-level, though the HP8 is also above average in this respect
• The HP8 imbues music with a certain beauty of tone, slight warmth, and palpable "thereness" that is quite alluring. IMO this is the tradeoff for the Aegis' greater clarity and articulation
• Stock tubes of the Aegis sound very good—tube rolling is not a necessity--while the HP8 can benefit from the best NOS tubes

I would love to revisit this comparison after systematic tube-rolling on the Aegis (I can dream).

CONCLUSION
The Aegis is the best tube amp I've ever heard—maybe the best amp, period. This is big-league tube amp sound worth seeking out.