METHODOLOGY AND GENERAL DISCLAIMER
 

 
I did not do any blind comparisons with this amplifier. My observations are subjective and biased. Nothing in this review should be trusted. I am not a professional.
 
I used the equipment mentioned in this review to evaluate the amplifier and I compared it directly with the equipment mentioned in this review. Most of my comparative listening experience (i.e. in the same environment, with the same software, with the same equipment in the stream) is on a HeadAmp GS-X (mk II modules), Oppo HA-1, or Inspire IHA-1. There is a huge and broad world of equipment out there. Any generalizations I make are based on a narrow slice of that world.
 
I will update the review as details or impressions change. I do not anticipate violent changes in perception, but I'm still getting to know this amplifier.
 
PACKAGING, FORM FACTOR
 
My Ragnarok was delivered ahead of schedule via FedEx. The amplifier is nestled inside a plastic bag and protected by two foam inserts. Also joining it in the box is an owner's manual and power cord. The box is snugly double-boxed, and while my external box suffered a bit in delivery, the internal box and amplifier were both immaculate. Removal from the box is easy and the packaging material is high-quality and worth retaining. Since the amplifier has decent size and mass, the packaged amplifier is somewhat large and heavy. Compared to most headamps and DACs it is a bit unwieldy. Compared to comparable integrated and power amplifiers, it is not particularly large or heavy.
 
In fact, I find the amplifier compact for its mass (dominated by the giant transformer). Positioning it for wiring is not a two-person job like it can be with larger and heavy power amplifiers. As with many aspects of this amplifier, expectations need to be adjusted: This is an integrated amplifier. It is quiet enough to drive virtually any headphones. It has a broad range of gain and great delineation of gain, allowing not just use in that regard, but viable use. That's a lot of capability in one box, and it costs in several dimensions. Look elsewhere if size and weight are dominating concerns over capability, but I don't think you're going to easily find many other examples of this much capability in a single compact box.
 
VISUAL AESTHETIC, BUILD QUALITY
 
Schiit carries a common visual aesthetic through its products. It's designed to keep costs down, look decent, and have practical (i.e. heat dissipation) benefits. I think it looks good, if a bit utilitarian. It doesn't feel cheap but it also doesn't feel like a luxury hifi product. Does this particularly matter? Depends on the purchaser, I guess. For me it's not a big deal: I'm here for the sound. Provided a component is solidly built and not outright ghastly, I'm satisfied so long as it sounds good.
 
For Schiit's higher-priced components, the exterior is wrapped on three sides by a rolled aluminum shell. It appears at first to be a single continuous piece but in fact is made in two parts, with a seam tucked into the bottom panel. This seam is only obvious when looking at the amplifier from the side or bottom, which isn't too common. The wrap is substantial and thick. The finish of the aluminum is a bit rough and the edges are sharp. I would not characterize contact with the edges as pleasant, but I also wouldn't class it outright dangerous. The most irritating instance of this is the edge around the control buttons, where fingers are likely to interact more often. It's not a big deal, but it feels a little cheap and unfinished. 
 

 
My biggest industrial design gripe is the volume knob. It has a small indicator on the front. It has a hole for a set screw. The set screw hole is far more distinct and large than the indicator. I would rather either do away with the indicator (just leveraging the set screw for indication) or better hiding/disguising the set screw hole. The set screw hole in my example also has a burr that is unpleasant to touch. Again, think this just makes the amp look and feel a bit cheaper than necessary.
 
The jacks, connectors, power switch, and inlet are all solidly affixed. These things all look pretty parts-bin, but this is not an unfamiliar appearance to those familiar with Schiit's industrial design.
 
The bottom of the amplifier features screwed-in feet instead of the adhesive stickies present in cheaper Schiit products. They are fine, though users wishing for better damping, isolation, coupling, or relief may be inclined to use tweaks and aftermarket products.
 
OPERATION
 
The volume knob is novel. It uses a potentiometer and has hard bottom and top stops. The potentiometer's level is interpreted by the control circuitry to set gain via relay-switching resistors. When spinning the dial the amp (itself, not the sound coming out of it) sounds a bit like the smoke monster from Lost. My significant other thinks it sounds terrible and like something is broken, but I find it charming. With that said, if for some reason an amplifier making clicking noises when changing gain is a problem, look elsewhere. I am not in love with the knob feel, but it does not feel at all loose or under-damped. With my headphones, speakers, and sources, I find a decent amount of gain has to be dialed in before the sound becomes audible. With that said, I find there is plenty of precision available to find-tune the level once I reach the desired gain. I've never run into the "but I really want the level in between those two clicks" problems that can sometimes plague stepped attenuators.
 
There is a decent amount of conversation in the forums about the heat of the amplifier. When I was using it at home with the Reference 3As, it barely got warm. It was on the top of a stack with good ventilation and in a room with good air circulation. At the office, it's under a desk in an equipment rack with about three inches of headroom. With the LS50s and HE-6 plugged in and it just idling, it gets toasty. The volume knob (and rest of the user interface) never gets dangerously warm, but it trends into the somewhat unpleasant range. I raised the amp slightly with a set of vibrapods and cones; this seemed to have a dramatic effect on the temperature. I am not sure why this is as there are no intake vents under the amplifier. My best guess is that a significant amount of the heat is radiated through the bottom of the chassis. Having a little more air helps dissipate this (and diminishes the heat soak into the shelf below). Incidentally, because of the uneven weight distribution in the chassis, just propping it up on vibrapods (at least up to #3) is going to make the amp sag to the left. 
 

 
Here's the thing: Compared to a large power amplifier, the Ragnarok does not generate that much heat. If anything the actual waste heat output at idle is similar to a small incandescent bulb. What is different, I think, is that there aren't huge heatsinks evacuating the heat from the chassis because the chassis is the heat sink. And, while it does a good job of spreading the heat out, it doesn't seem to be quite as effective at evacuation as a large finned block of aluminum (especially those mounted to the outside of the amp). That said, there are plenty of large fin-happy power amps (especially pure-class-A designs living snugly within their thermal design) that feel like they could cook dinner. The Ragnarok has vents over the transistors but a lot of heat seems to be radiated directly through the chassis itself. Schiit designs its products this way on purpose and it's a functional approach. What it does not yield is a cool-to-the-touch user interface panel.
 
At the end of the day, I don't care how warm the amp gets. It's an amplifier and I'm not going to rest my head on it while trying to nap. It generates heat. It doesn't get hot enough to be dangerous. But, if a warm-to-the-touch amplifier is something that makes people uncomfortable, it's probably best to look elsewhere. If wasting power isn't somebody's thing, same thing. I would be cautious about assuming that a cooler-running amplifier with similar circuit design and specs is less wasteful, however; it's likely just better at getting the heat away from the amp.
 
The power switch is on the back of the amplifier. This makes sense in practical terms, I presume (since it's next to the AC inlet), but it is somewhat inconvenient. Those not desiring always-on operation may find this troublesome. With that said, the amp should have a decent amount of vertical airspace above it for ventilation. Any reasonable interpretation of this will make it easy to reach and throw the switch.
 
Switching inputs is simple and straightforward. While not a new trend, I appreciate they aren't labeled things like "CD" or "tape." Just simple numbers, 1 through 5. Solid.
 
Setting the gain level is similarly simple and straightforward. Hit the gain button and the appropriate indicator lights up. The gain button is on the other side of the volume knob as the indicator. This looks nice but seems a little visually confusing -- where the input selection button is adjacent to the input indicators, the gain button is on the opposite site of the volume control. 
 

 
This brings us, then, to the most frustrating and inexcusable thing about Ragnarok's user interface: output selection. Holding the input button down for a long push switches the output mode of the amplifier (headphones + speakers, speakers, headphones). There's no way to know what mode the amp is in at any given time just by looking at it. For people that always operate it in a particular mode, this is a non-issue. I switch modes regularly and it drives me nuts. Adding one more button and two LEDs (headphones hot, speakers hot, light both up for both hot) would be a tremendous improvement. I am happier to have this approach than an auto-sensing switch (i.e. when I plug in headphones the speakers mute), but that's about the nicest I can say about it. If there's a minor (or major) hardware/chassis revision to the amp, this needs to be addressed. I would prefer the gain setting/button as the "hidden" UI if room had to be made.
 
AS A HEADPHONE AMPLIFIER
 
Enough with the unimportant stuff, let's talk about sound. I'll cut to the chase: Ragnarok is a good universal headphone amplifier with neutral sound profile. Ignoring electrostatics, it drives anything I could plug into it well.
 
First off, let's talk about noise. There isn't any. Ragnarok is dead-quiet at all gain levels with all the headphone devices I've tried. According to Stoddard, this is an amp that is sensitive to noise on the AC line but I haven't run into any problems on this front. Maybe my AC just isn't that noisy. My Ragnarok sits behind a high-current power conditioner (20A) and a moderate-current isolation transformer (10A). I didn't find an appreciable difference versus just plugging it into the wall.
 
Power? It's got it. The amp can definitely drive the HE-6 to loudspeaker levels. Easily. In low gain. In fact, I've only used the gain selector switch to verify it works, I leave the amp in low gain at all times. See rant above about my wish that it was replaced with the output selection system. NB I'm driving the amplifier with balanced input and relatively high-voltage ones at that. Those using low-voltage sources and/or unbalanced signals may need more gain from the amplifier itself. This isn't necessarily a good or bad thing, but it's a thing. Where one's volume control ends up and whether or not they need a gain setting has more to do with the upstream component chain than anything else.
 
Control? Check. Without it, the power's useless, after all. It does well with high-impedance loads. It does well with low-impedance loads. It does well with highly-sensitive loads. It does well with insensitive loads. 
 
Delicacy? No problem. Plug in those sensitive CIEMs with aplomb. It's all going to be alright.
 
I have a hard time describing the sound signature of the amp as anything but neutral and clean. It reproduces music reliably and faithfully. It's not warm. It's not cold. It's not sterile. It's not dark. It's not rainbows and unicorns. It resolves plenty of detail but never comes across as harsh or fatiguing. It doesn't sound much different whether it's on for a minute or a day. It doesn't seem to have changed in sound signature over the weeks I've owned it. 
 
Some of the forum feedback is that the amp has tube-like characteristics or sound. I don't know that I'm in that particular camp; while I have zero experience with OTL tube amps, direct comparisons with the WA7 and IHA-1 leave me feeling the amp is not particularly tube-like. It doesn't seem to be coloring the music with pleasant (or unpleasant) harmonic distortion. It doesn't seem to be an off-neutral presentation of tone with "great tube bass" or "tube-like warmth" or any other common tube epithets. In fact, I would argue the amplifier sounds like wire with gain more than anything in particular.
 

 
Best headphone pairings? Hard to say. I'll start by talking about the three which with I've spent the most time so far, but I can't say that anything thus far sounds less than "good" with the Ragnarok. That's a bigger compliment than it sounds.
 
Probably the closest to "match made in heaven" is the HE-6. Nothing else in my stable controls it as well. And, with the HE-6, that is always the challenging part: Enough power and good control of that power? Check and check. I like the HE-6 with relatively neutral amps and signal chains and feel like it is capable of magic under the right circumstances. This combination does not disappoint. Run, don't walk.
 
The HD800 is a fickle creature in a very different way than the HE-6. I'm happy with the combination, but I'll be honest: it's a very neutral amp with a set of headphones that strike me as somewhat clinical at times. The pair is not kind to poorly recorded or mastered material. Detail and resolution is fantastic, but the pair is far kinder to the mid range and human voice than software with a ton of high-frequency or low-frequency material. For folks that love the HD800s with solid state amps, Ragnarok does not disappoint. For folks that love warm tube amps with HD800s, this is not a warm tube amp.
 
The LCD-3F does rather well. I find these headphones sometimes end up a little bass-heavy or treble-light (or both) depending on the amp. Thus, I like the presentation of the Ragnarok here. Bass is controlled and present but not over-emphasized. Treble isn't missing but also isn't excessive in any way. I think this is a solid performance.
 
a note about the single-ended output
 
The single-ended output of the Ragnarok is provided, I believe, as a convenience feature. Since the amplifier is balanced end-to-end, a hardware signal summer is necessary in order to produce a single-ended signal (running single-ended termination out of the balanced output is bad news; on the upside it will safe the amp rather than do bad things). 
 
I have identical balanced and single-ended cables for my PM-1 headphones, so I took the time to verify single-ended operation and see if there were any obvious differences. While I don't think cables are at all a dominating factor, I can't say precisely the same for any headphone I own otherwise -- the cables are different. I'll note that the PM-1 is an incredibly easy planar to drive and that, like with much of this review, this is anecdotal. Results may vary and all that jazz; notably the output level is much higher through the balanced output, so level matching is not guaranteed when trying to A/B.
 
So with all of that disclaiming out of the way, my initial take is that the single-ended output sounds fine, but not quite as tight as the balanced output. Most notable in attack/release of lower-octave material. Confirmation bias may be at play here, but I think you're giving up some of what you've paid for if you're only using the single-ended output. If you're primarily using this as an integrated amplifier with the headphone feature as a convenience, your mileage may vary.
 
AS A PREAMPLIFIER
 
I don't know how Ragnarok fares as a preamplifier. I have not tried to use it in this capacity. I intended to and I forgot, and now I don't have a convenient amp nearby. "The manual says it should work" is about the best I can offer.
 
AS AN INTEGRATED AMPLIFIER
 
Let's pretend for a moment that Ragnarok is just an integrated amplifier. Here are two concrete anecdotes:
 
Compared to a push-pull valve amplifier at a similar price point and power output (Rogue Atlas Magnum, unbuffered stepped attenuator, unbalanced operation, short low-capacitance cable runs), I think the Ragnarok fares pretty well. It has great detail and neutral response across the frequency band. It doesn't sound like this tube amp, but I have a hard time saying that the tube amp had the upper hand in any particular area beyond looking amazing in the dark. It's a bit of a different animal, but not better. I have a romantic fondness for the tubes here, but I feel the Ragnarok is just a far more honest interpretation of the music.
 
Compared to a class-AB solid state amplifier at a higher price, class-A bias, and power output point (Bryston 4BSST2, unbuffered stepped attenuator, balanced operation, short low-capacitance cable runs), The Ragnarok does respectably but is situationally bested. With a pair of Reference 3A loudspeakers, I have a hard time saying I preferred one amp to the other. With a pair of KEF LS50 loudspeakers I have to give the edge to the Bryston. These are not speakers with dramatic bass extension to begin with, but I preferred the Bryston's control of frequencies below 100Hz. Despite this, I'm leaving my Ragnarok wired to the LS50s. For my needs in that particular system, the benefits of convenience and simplicity are trumping dragon chasing the ultimate in sound quality.
 
Beyond that I have a lot of short- and long-term memories of equipment, but I don't think those should be considered reliable.
 
Generalizing these anecdotes is a tenuous exercise at best. About as far as I'm comfortable going at this time is that I believe the Ragnarok represents a great value as an integrated amplifier, too. This amp deserves serious consideration for anybody shopping for an amp/integrated at this price/specification point, whether they care about headphones or not.
 
ALL TOGETHER NOW
 

 
Here's where I end up on balance: Ragnarok provides a capable integrated and universal headphone amplifier. It provides the ability to drive sensitive and demanding headphones with control and low noise. It provides a great range of volume control for each of these scenarios. This is a tremendous amount of capability and versatility in one box. The amp is a jack of all trades. Is it the master of none? The end of the world? Considering it can drive many speakers and virtually any non-electrostatic headphone well, it's a steal at the price point. Want to run headphones or speakers, spend less than $2000, and spend zero time thinking about it or worrying which headphones will work? Buy this amp. Want a speaker playback system and want to spend about $3200? Pick up a pair of LS50s and call it a day. Want to try to optimize for a particular load, headset, or capability? Probably a somewhat more difficult exercise. This is a great starting point, but there may be some other options to consider depending on one's listening preferences or designs on space, heat, or capability.
 
SOME OBVIOUS QUESTIONS WITHOUT ANSWERS
 
How does this amp sound versus the Mjolnir? I have no idea. I don't have a Mjolnir. I wanted a single box that could provide both the integrated amp and headamp capabilities. For that matter, I have not done in-depth comparisons with any headamps with the same environment and gear except for the GS-X, IHA-1, and HA-1. Versus the Mjolnir, this amp can deliver more power, can drive speakers, takes a different approach to handling bias, has more inputs, has single-ended output, and has a fancy gain attenuation system. It's also heavier, bigger, and costs more. That's about all I know.
 
How does this amp sound with the HE-6 versus plugging it directly into the back of a First Watt J2 or F1J? I wish I knew! If you would like to send me yours I will tell you. What I won't do is promise to return it. In general, see the above. I don't know and my speculation isn't going to be useful.
 
How does this amp sound with a pair of Fostex TH-900s? I don't know. I don't own them. With a Beyerdynamic T1p? Same idea. No clue. Abyss? See offer about the First Watt above. If it's not in the list of equipment below, I haven't tried it.
 
ASSOCIATED EQUIPMENT
 
The amp was evaluated in two different environments, so it's a bit of a mess:
 
Power Cables: Pangea AC14SE, Stock Cable
USB Cables (don't make a difference, but they look pretty): Wireworld Ultraviolet
Line-Level Cables: Kimber Kable Timbre (balanced), Better Cables Silver Serpent (unbalanced)
Phono Cables: Kimber Kable TAK-Cu
Power Conditioner: Furman P-8 Pro Series II + Torus Power IS10, Torus Power RM15Plus
Digital Chain: Macbook Pro, Audiophileo1/PurePower, NAD M51 (balanced), Schiit Bifrost, Schiit Wyrd
Analog Chain: Sumiko Blackbird, Pro-Ject RM-10.1, Simaudio Moon 320S/310LP (balanced)
Headphones: Sennheiser HD800, Audeze LCD-3F/LCD-XC, Hifiman HE-6, JHAudio JH13Pro, Oppo PM-1
 
CRITICAL LISTENING SOFTWARE
 
The following are the recordings I focused on when listening critically; have put a lot of additional software through the amp when I was just listening to the music. Except where noted, I auditioned each in both vinyl and red book from my own CD rips.
 
Opeth - Pale Communion, Ghost Reveries                           
Porcupine Tree - Lightbulb Sun
Steven Wilson - The Raven That Refused To Sing
Nine Inch Nails - Hesitation Marks (Digital copy is direct FLAC)
Aphex Twin - Syro
Stravinsky - The Firebird Suite (Reference Recordings, analog only)
Miles Davis - Sketches of Spain
 
UPDATES
 
20141211 - added some commentary on single-ended headphone output; fixed several typographical quirks
20141214 - a little commentary about how the required gain may vary with upstream components
 
TODO
 
Things I definitely want to do, beyond revise this as more time develops:
 

1. Specific feedback on JH13Pro, PM-1, LCD-XC?

BACKGROUND 
 

 
You may not have heard of Dennis had, but you've likely heard of his products.  Dennis Had and his wife Donna founded Cary Audio Design in 1989, but the story I've been able to piece together (go Google a bit, I'll wait) is also rich and fascinating. As he mentions in his eBay listings, he built his first single-ended 6V6G amplifier for a 7th grade science fair and has been hooked ever since.
 
Had left Cary a few years ago, but he continues to design and hand build single-ended tube audio components as a hobby. His "retirement hobby" is our gain. Had releases a handful of his designs on eBay semi-regularly (I've seen quoted a figure of 1-2 per week but have seen far less than that recently), usually consisting of amplifiers (stereo and mono pairs), integrated amplifiers, and headphone amplifiers. True to his original beliefs, they are all single-ended designs focusing on simplicity and the listening experience.
 
Had's affection for single-ended designs are well-explained in a 1995 Stereophile interview:
 

More than anything, the virtue of single-ended is the simplicity, and that the signal is handled in as linear a fashion as possible. In a push-pull amplifier you split the signal into two halves and then recombine those halves at the amplifier output. I felt that the power amplifier should just replicate what the CD player or turntable is outputting and maintain the integrity of the waveform. Single-ended makes the most common sense from an engineering standpoint and also has the least number of parts in the signal path. You're not taking the signal and dividing it up.

Click to expand...

 
THE AMPLIFIER
 

 
The Dragon IHA-1 is a custom version of the Inspire IHA-1 offered irregularly on eBay. It is distributed exclusively through Moon Audio and as I understand it was inspired by Drew's introducing Dennis to some Audeze headphones. There's a quick video with Baird and Had about the amp if you want to learn more at Moon Audio's site.
 
The Dragon Version has some changes versus the model that most commonly appears on eBay. I'm not aware of the internal changes if there are any (and I don't have the heart to crack mine open), but externally some differences are readily apparent. First, the front plate is black and the paint job is a copper-red metallic finish that would feel at home on a vintage American muscle car (versus walnut veneer on white). Second, the LED is blue instead of read. Third, the output jacks are upgraded, with professional locking 1/4" and 4-pin XLR connections (this is a single-ended design, but the 4-pin jack takes its connections from the output taps of the output transformers; my listening has been exclusively via the 4-pin jack). These take the place of a pair of 1/4" inputs and are mounted to the chassis rather than through the faceplate. Fourth, the potentiometer has been replaced with a dual DACT stepped attenuator.
 
On the back of the unit there are two pairs of RCA inputs, a fuse holder, and an IEC inlet. The amplifier comes with a standard power cable.
 
The design leverages dual triode 6SN7 tubes in class A single-ended mode with zero feedback. Had recommends the (lower gain) 6BX7 but any 6SN7/SBL7 should work (e.g. the RCA 5692). The tube rectifier is 5Y3/5U4/274B types as well as 5AR4, though Had recommends against indirect heating in this application.
 
The design is direct-coupled and output is via custom air-gap output transformers. It produces just over a watt into 32 ohms and a bit under a watt near 600 ohms. That should make it powerful enough to drive most loads.
 
AESTHETICS & OPERATION
 

 
Make no mistake, this is a low-production bespoke product: The fit, finish and polish are good but this unit will not win any design awards. The front panel is connected to the chassis with some sort of adhesive and bits of hair and fuzz around it remind you of that fact under any close scrutiny.  The output transformer sheet metal bends aren't particularly uniform and the labels on top of the transformers are crooked. The RCA inputs in the back aren't labeled 1 or 2 (they do have channel indicator rings, however), though it's easy to figure out which is which. There are at least four different styles of type on the front face and they look like they were placed without any real thought to design and layout. The amplifier's packaging is "toss it in a box wrapped in 100 feet of bubble wrap. The entire bottom of the amplifier is covered in Dennis's scrawl, except for the part obscured by the rubber foot (no joke). I happen to thing it's a nice touch on a very personal product, but others may not. The blue LED looks sort of like an awkward afterthought; I'm pretty sure I prefer the red of the stock unit. Those looking for the attention to detail and incredible fabrication of e.g. a Woo or HeadAmp will sneer in disgust. It's not that the Dragon IHA-1 is outright ugly, but it looks a lot better in the dark.
 
The tube sockets are solidly anchored; I have no concerns when changing valves. For that matter, everything about the amplifier is solid. The transformers aren't going anywhere, the RCA jacks are firmly installed, and there's no play, wiggle, or shake in the amp. Despite the entire previous paragraph, everything mechanical and functional about this amplifier is clearly sorted and well thought-out. Had may not be a champion of the visual aesthetic, but his design shows mastery of the physical and functional. The input selector switch has a firm and positive throw. It's never going to be triggered by accident and it's obvious when a change has been made. It's intuitive that going left is the left input and going right is the right input, and that mute is in the middle; operation of the amp in pure darkness reminds the listener that this amp's ergonomics were designed for somebody using it ... not somebody looking at it. The attenuator knob has perfect knob feel and was left pleasantly small. For me this is important; I find large knobs to have enough size and mass that positive identification of detents becomes difficult as attenuators age. The rectangular shape of the knob makes it easy to identify and turn with a couple of fingers in the dark.
 
I'm not a huge fan of locking connectors for headphones. They feel great, but they increase the risk of destroying a cable, headphones, or the amp itself if I do something clumsy (I regret this happens more often than I'd like).
 
The form factor itself is deceptively small; it's about 2/3 of the depth and width of a typical (~17") stereo component. This isn't that weird since there are a lot of goofy sizes in head-fi. The unit has tall feet and can take up a lot of vertical space when coupled with a large rectifier. The layout is rather compact with the three tubes, capacitor, and three transformers all packed together in pretty close quarters. The three transformers provide a substantial amount of mass, but I would not call the unit heavy as much as solid.
 
Operation is easy: Plug in the tubes, flip the on switch, wait a few seconds, roll the input selector, and listen to some tunes. At least, I think that's how it works as there aren't any instructions included. I can imagine Had saying "why would you need any instructions? I made this thing as simple as possible, you can't mis-use it!" Or, maybe not, but that's what I'd like to imagine.
 
Included by default are a Sovtek 5Y3GT rectifier tube and a pair of EH 6SN7s. In that trim, the amp runs $1599. The upgrade tubes optioned are NOS Sylvania 6BX7 output tubes ($100) and a Sophia 274B ($160) rectifier.
 
For those new to tubes, the output tubes don't look like much when they're running; in some cases (like the Zalytron 5692) the getter in fact completely hides any sort of illumination. They can run hot, however, with surface temperatures around 250 degrees. There's no tube cage on this thing, so be careful. The rectifiers range from mildly interesting-looking to downright gorgeous under operation; the giant Sophia tube is more in the gorgeous direction, with its bright direct heaters and stamped "mesh" plates. It's nowhere near as warm on the surface and could probably be handled during normal operation if somebody was particularly weird.
 
With both of my signal paths, I find I'm using the volume knob roughly from 9 to 3 with the majority pretty close to noon. I feel like there's an appropriate level of gain gradation in this range and don't feel the amp suffers from the stepped attenuator plague of wanting a setting in-between a particular set of detents. Where in other amps with stepped attenuators I often have problems finding the goldilocks level, this has not been a limitation in use with the HD800/LCD-3F.
 
THE SOUND
 

 
This brings us to the only thing that should really matter for those considering the purchase of this amplifier: How does it sound? Well, that's a good question! I'll offer my initial impressions, but will caution that these are at best preliminary: I reserve the right to update this as I have more time with the amp, roll more tubes, and rotate more headphones through it. 
 
I've spent about 20% of my time listening to the amp with Sennheiser HD800s and the remainder with Audeze LCD-3Fs. Both cans were wired with Moon Audio Silver Dragon cables and terminated with 4-pin Neutrik connectors. I did not (and indeed may never) try the 1/4" output yet. I like both, but I love the Audeze cans on this amp especially; the pairing is special.
 
Before I continue, one final disclaimer: My desktop headamp listening experience is most extensive on the HeadAmp GS-X (mk II modules), Woo WA7/WA7tp, and the Oppo HA-1. When I used blanket generalizations like "compared to other amplifiers" I'm referring more to this canon than the entire universe of headamps. 
 
It's for me somewhat difficult to describe how this amp sounds; the easiest way I can really describe it is that it's a very good lens through which one can listen to music through a certain set of tubes. If the tubes sound a certain way, the amp sounds a certain way. In rolling several different valves I never latched onto a sound signature of the amplifier itself. I don't mean to be evasive about this, but I remain struck by a notion that this amp is all about getting out of the way and letting the tubes do their thing -- good or bad. 
 
So, the problem then is I have a hard time latching on to the common refrain about such a design that the amp sounds warm. With certain tubes it sounds a little warm, I guess, but I don't think warmth is an intrinsic property of this amp. Instead I would describe the sound signature of the amp and tube configuration having more the characteristic of "not dark" than "warm." The presentation strikes me as having a neutral mid-range, with austere bass and neutral to just shy of neutral treble.  While the bass does not have incredible slam and impact, the precision of the attack and release more than make up for it: There is plenty of low-range extension and it doesn't leave me thirsty. The low-end signature of the amp reminds me of that of the HD800s; the pair combined amplifies the effect. Whether or not I'd call it "accurate" bass I think the word that comes to mind most is "realistic."
 
I rolled with a few rectifiers and half a dozen output tubes, but fell into immediate love with the pairing of a Sophia Princess 274B (from Moon) and a pair of CBS-Hytron 5692s (from Upscale Audio) and haven't changed since. I wanted to love the Sophia 6SN7s just because they looked amazing next to the 274B, but it just wasn't meant to be. Stop looking when you find the one you love, or something like that? So, all of the feedback about the amplifier in this review comes from the Sophia/Zalytron pairing unless otherwise noted.
 
Soundstage remains a nebulous concept for me with headamps: I wouldn't describe the presentation as particularly broad or narrow versus other amplifiers. Positional clarity or lack thereof similarly is not particularly unique or distinctive. The word that does come to mind is that the audio presentation is "holographic." It's not that the performers are suddenly in front of me on a stage and I can tell you how far away they are and how far they sit apart; those are the terms and sensations I still reserve for loudspeaker systems. Instead, the sound itself is incredibly three-dimensional and provides a great deal of texture and depth. I'll use the holographic term as somewhat of an analogy: A lot of paintings have a great deal of depth and texture and intrigue, but they look the same no matter how the light strikes them or how they're tilted. The hologram has more to it than that; one finds different things depending on what they're paying attention to and it can be somewhat overwhelming to try to take it all in at once. Good music is confusing and overwhelming and rewards careful as well as casual listening. This amplifier is not a scalpel for surgical analysis of music. It seems to be more designed to step back and say "look, there's all this crazy stuff going on that you never realized before!" Wow. Give me a few minutes...
 
... dear reader, I got lost listening to the amp for a few more hours, but I'm back again. Sorry about that.
 
The holographic sound is what grabbed my attention with this amp, but I want to talk a little bit more about what retains it: More than anything, the microdynamic capability of this amplifier is astounding. I have not been exposed to an amplifier that delivers better microdynamics.  To all of those folks ready with the pitch forks, there are plenty of amps I haven't heard yet, especially in the high-end valve realm. The texture and detail capability is impressive. It doesn't matter if I'm getting slammed with loud beats, a pin drop is still distinct. The speed and precision of the amp is incredible. I listened to songs I've probably heard at least a thousand times, and with each one I was discovering new depth and richness of sound that I never experienced in the past. This didn't just happen when I was listening for it; the amp has a way of grabbing your attention and saying "listen to this, just get lost in this passage, you've never heard it like this before..."
 
... sorry, got distracted again. I'd say I lost a few hours of my life, but it was definitely a gain.
 
If folks are interested I can try to comment on other aspects of the sound quality, but for me this part is the kicker: The amp is incredibly euphonic for me for the reasons listed above (holographic sound and microdynamics). It makes me want to listen to the music and it makes it incredibly hard to listen to the equipment. If there's a dragon to chase in head-fi, that's mine. I hate listening to equipment. Listening sessions that end with "I wish this X had more Y" just fill me with disgust. I'd rather end up with "okay, just one more track..."
 
It could be said that everything I'm hearing is just second harmonic distortion and I'm in love with distortion and not signal. To that I say "that's entirely possible!" But, I enjoy it just the same. There's plain and there's French Vanilla, and I'm enjoying the French Vanilla with this amplifier. There is probably something intelligent that could be said about how the amplifier lacks clinical accuracy and 100% authentic reproduction, but I'm not the guy to say it. Count me as a believer in whatever it is this thing is doing to the music. 
 
MORE OF THE SAME
 
The capabilities and pleasure of the amplifier have continued as I've expanded beyond the HD800 and LCD-3F. 
 
First up, the Oppo PM-1 puts on a good showing. These headphones are surprisingly sensitive, so the full range of the attenuator knob is not necessary. Volume levels quickly get into uncomfortable territory around noon or so. Still, I have not had any difficult finding the "right" volume level in the gradations available to me before 1 or so. I like the LCD-3F with this amp more because I like the LCD-3F more, but I think the PM-1 shows very well here. Due to their high sensitivity, Oppo's headphones make the low-level hum in the right channel more obvious.
 
Given how the LCD-3F sings with this amp, it should not be much of a surprise that the LCD-XC also pairs rather well. I don't have too much to add beyond that they sound a bit more closed than the 3F. Go figure, right? Since they are closed, they also make the low-level right-channel hum ever so slightly more obvious when there is no signal present.  
 
PLAYING WITH THE EXTREMES: A MIXED BAG
 
Happy with how the HD800 sounds and very happy with how the LCD-3F sounds, I wanted to explore the amp with some of the more extreme cases I had in my inventory. In the grand tradition of "Will it blend?" and "but does it play Crysis?" we must also confront whether or not the amplifier can drive the HE-6. With the 6BX7 I have to max the gain to get to a "normal" listening level. The 6SN7 pulls things up just enough to have a couple of detents to play with, but I would have a hard time recommending this as being the go-to amplifier for the HE-6; it's just not going to get loud enough for many with that particular set of headphones. With that said, for the volume levels the amp is capable of, the headphones do sound great. So, more sensitive Hifiman models (I have no personal experience) may work better, or this could still be a great pairing for those that don't listen too loud.
 
At the other end of the spectrum, I plugged my 4-pin-terminated JH13Pros into the amp. I would not recommend this particular pairing; the general noise levels are too high for my taste with this particular in-ear monitor.
 
Back in a very different direction of hard to drive, I plugged the AKG K701s into the 1/4" jack (good news, I can now confirm it works too). The amp is able to drive these cans and there's still a little room to play in the high end of the gain (more so than the HE-6). With that said, the sound lacks a bit of control compared to e.g. the GS-X in some quick comparisons. The pair sounds decent, but a little thin for my tastes.
 
NITS AND CONS
 

 
Let's talk about the downsides. Optioned out with some tubes for rolling, this amp is bumping up against two thousand dollars in the US.  That's endgame or well beyond endgame pricing for a headamp. It might be somewhat difficult to call anything in that league "a value." I would not say the amp is embarrassed by gear at a similar price point. To sum up, on value my feeling is that the amp punches within its weight class and I'm rather taken with it. For those interested in getting more bang for the buck, the eBay IHA-1 lacks the upgrades mentioned above, but it also comes in a bit cheaper at $1200. At the end of the day, this is an extremely low-production hand-built product and one is getting both the benefits and detriments of that.
 
I think I've already beaten the amp up a bit in terms of looks and fit/finish so I won't go into that point further.
 
It comes with zero functionality beyond being a two-source unbalanced headamp. There's no line out, pre out, or speaker out. There's no balanced inputs. There's no gain switch (the only real option to adjust there is in tube choice; the 6BX7 provides less gain than the 6SN7 types). About the only display or indication is the blue LED. I tend to prefer this sort of simplicity of focus and purpose, but it's important to keep in mind this terminates a signal path in one's system and nothing more.
 
Finally, there is a very mild hum in the right channel; it starts when the amp is turned on and rapidly attenuates after the first few seconds of amplifier operation. After 15-30 seconds it's virtually inaudible and incredibly difficult to hear even with quiet passages in the music. Had responded to another member explaining this in detail in this post. I'd recommend reading that for any concerns on this point. I think this would only bother or impact people with extremely sensitive headphones/in-ear monitors, and it's really only noticeable when there's no signal present otherwise. I don't tend to listen to headphones without a signal present, so this has not been a problem for me! Had clarified in e-mail that it's down about 78dB from the music signal, so when I say mild, I mean mild. I was not clear on whether it's because of the circuit design or the physical layout; I attempted to get clarification on this point but did not hear back from Had. In my experimentation, this is less noticeable when using 6BX7 tubes than 6SN7; several other head-fi folks only notice it with the 6SN7. I did experiment with different power cords, isolation transformers, power regenerators, and other voodoo devices. None had any particular impact on this hum.
 
ASSOCIATED EQUIPMENT AS REVIEWED
 
Power Cables: Pangea AC14SE
USB Cables (don't make a difference, but they look pretty): Wireworld Ultraviolet
Line-Level Cables: Kimber Kable Timbre (digital: Ultraplate, analog: WBT)
Phono Cables: Kimber Kable TAK-Cu
Power Conditioner: Furman P-8 Pro Series II, Torus Power RM15Plus
Digital Chain: Macbook Pro, Audiophileo1/PurePower, NAD M51
Analog Chain: Sumiko Blackbird, Pro-Ject RM-10.1, Simaudio Moon 320S/310LP
Headphones: Sennheiser HD800, Audeze LCD-3F, Hifiman HE-6, AKG K701, JHAudio JH13Pro
 
CRITICAL LISTENING SOFTWARE
 
The following are the recordings I focused on when listening critically; have put a lot of additional software through the amp when I was just listening to the music. Except where noted, I auditioned each in both vinyl and red book from my own CD rips.
 
Opeth - Pale Communion, Ghost Reveries                           
Porcupine Tree - Lightbulb Sun
Steven Wilson - The Raven That Refused To Sing
Nine Inch Nails - Hesitation Marks (Digital copy is direct FLAC)
Aphex Twin - Syro
Stravinsky - The Firebird Suite (Reference Recordings, analog only)
Miles Davis - Sketches of Spain
 
FULL DISCLOSURE
 
I am a Moon Audio Ambassador, but that role has nothing to do with the purchase or evaluation of this amplifier. I purchased it with my own funds and reviewed it independently.
 
UPDATES
 
20141115 - a few clarifications, fixed a few typographical goofs.
20141117 - added a few more headphones, clarified current state of noise understanding
20141117 - further clarified hum concern, linked to BlakeT's information on this (thanks!)
20141118 - added a brief section about the PM-1, LCD-XC
20141130 - minor wording tweaks
20141207 - minor wording tweaks, type-o squashing, admitting I might just like second harmonic distortion
 
TODO
 
These are things I'm still intending to explore, and may provide updates in these areas over time:
 

1. Tube rolling!