Niimbus Audio US4+


Headphoneus Supremus
Pros: Superb transparency, low end control and texture, massively powerful yet delicate enough for IEMs, does double-duty as a high-end preamp (including remote)
Cons: Very expensive, polarizing looks, lacking emotion compared to the best tube amps out there, I wouldn't mind a few more inputs for preamp duties

Who makes better cars - Toyota or Lexus? How about Honda versus Acura, or Nissan against Infiniti? Of course, these are silly comparisons, as Lexus, Acura, and Infiniti are simply the luxury divisions of their respective manufacturers. The cars themselves often share a basic platform, with the luxury version shooting for more style, features, and brand cache. This succeeds or fails on a case by case basis but I can clearly see the point of it - the "Lexus" marquis allows Toyota to sell LS-series sedans for $75K+ when nobody would accept a Toyota-branded sedan in that price range, however nice it may be.

The audio industry occasionally does a bit of this as well, though it usually goes in the opposite direction. Established brand YBA went downmarket with their Audio Refinement line, ostensibly to reach new customers who couldn't afford their normal gear. Same thing with Raidho speakers and the more affordable Scansonic models, or Cary with their Audio Electronics line of gear (now defunct, as far as I can tell). All three of these examples had well-regarded designers and established firms putting out gear that just wouldn't fit under the original brand banner.

The best example I can think of going in the upward direction is German firm Lake People GmbH. Despite making studio-oriented equipment for several decades, the name still didn't carry much weight in certain regions; including the USA. So the company launched Violectric roughly a decade ago as a way of branching out into the high-end consumer market. Their line of headphone amplifiers, DACs, and preamps has earned many accolades, with the V281 serving as my reference headphone amp for the past several years. Most folks seem pretty comfortable considering Violectric as "high-end" gear, whilst more affordable offerings sporting the original Lake People branding reap the benefits of improved name recognition. It's win-win.

What to do when you want to move even further up market? One idea is to launch yet another brand name specifically encompassing the best of the best. That's Niimbus, the "Ultimate" series from designer Fried Reim of Lake People/Violectric fame. This is the statement line of products, with pricing - and hopefully performance - to match.

At the moment, Niimbus is comprised of just two upcoming products - the US4 headphone amplifier, and the US4+. One or more DACs will come later. Both headphone amps share the same core components, with the Plus going beyond in terms of pre-amp functionality and a few other bells and whistles. I've been spending time with a prototype US4+, so that's what I'll be discussing. I don't yet have a specific date for when these will be available other than "pretty soon". Also keep in mind that the appearance may end up changing - probably by a small amount, but there's a chance it may be substantial.


The very first thing to mention is the pricing - it isn't finalized yet, but I expect somewhere close to $4,000 for the US4 and $5,000 for US4+. Give or take a bit. That puts the Niimbus line up there with some of the most expensive headphone amplifiers on the market.

I'm not going to go too deeply into the pricing aspect. It simply is what it is. No doubt many can't afford it, and some will even be downright offended that it exists. I can absolutely understand that point of view.

I'll just point out two things. One, the US4+ is a very complex device. Lots of quality parts, and lots of labor to assemble it. Two, Violectric is based in Germany, and does all their assembly in-house. CEO Fried Reim has consistently upheld the view that he wants to pay his workers a respectable wage - a practice which is surely honorable no matter our thoughts on final product or pricing. It seems to have the added bonus of delivering superb quality control - you very, very rarely hear of a malfunctioning Lake People or Violectric device. Happy workers apparently do better work.

Offloading production to China would certainly allow for significant price reduction. As would a direct-sales business model. The company has always used the more traditional setup with regional distributors who no doubt require their cut. Again, we can argue about the merits of each approach, but in the end the Niimbus amps cost what they cost, period. Some people will find it absurd, a few will buy it without hesitation, and most will be somewhere in the middle.

So just what to we get for that cash outlay? In the case of the US4+, that would be a balanced solid-state amplifier representing over three years of development, which doubles as a superb (if somewhat light on inputs) preamp. Inside the case we find a massively beefed-up design only partially reminiscent of already excellent V281. No longer constrained by the V281's "shoebox" form factor, this larger enclosure houses over 60 transistors, twin shielded 25W toroidal transformers flanked by over 50,000uF worth of Nichicon filtering caps, a highly advanced volume control solution, and extensive protection circuitry. The result is an amp capable of swinging 32V into 600 ohm loads, whilst topping out at 7W per channel at 50 ohms.

Backtracking to that volume control implementation for a moment; the V281 had an optional relay-based solution featuring 128 steps in .75dB increments. It added nearly $600 to the price, and made a small but noticeable improvement over the base-model's Alps RK27. Niimbus amps will not have options. The sole offering is an exotic 256-step affair (in .4dB increments) based on reed relays - the contacts are situated in a sealed glass tube which is filled with gas and operated magnetically. This solution is exceedingly precise, with perfect channel matching at all levels and plenty of steps for fine tuning. It has the added bonus of being nearly silent in terms of physical operation. Some folks with the relay-equipped V281 complained about the audible clicks when making adjustments, though I personally never found it bothersome once I knew what to expect. The Niimbus volume solution is much less audible though - on a quiet day, with your ear next to the amp, you can hear just a tiny bit of noise, but that's about it.

Interestingly, the focus here seems not on additional power - though there is an increase - but rather the decrease of noise. Going back to the Violectric V200, and many other Lake People/Violectric models, the amplifier gain was 8dB. Balanced operation on the V281 adds 6dB for a total of 12dB gain. But when we get to the Niimbus, amplifier gain is -4dB... signals passing through the amp are actually lower at the output than they are at the input. In this way, it could be said that Niimbus acts more like an impedance converter than a traditional amplifier with gain. And when we add the 6dB for balanced output, we end up at a final value of 2dB - which is 10dB less than balanced out on V281. A 10dB reduction in noise is remarkable considering the V281 is already an extremely quiet amp. Niimbus continues the now-familiar pre-gain adjustment options, expanded in functionality over the V281, so final gain can be set to match very quiet or very hot sources while keeping plenty of travel in the volume knob.

Review System, or "It's Complicated"
I wanted to keep this simple. I really did. Short and sweet, just do a bunch of listening and report on my thoughts. But one thing led to another, and things kept snowballing until I had a ridiculous collection of gear involved. This is almost certainly the most complex evaluation I've ever done for a single product. In an attempt to salvage some small portion of brevity, I'm going to limit my comments to the Niimbus amp itself rather than the associated source gear. I may eventually write up my thought on the DACs involved, but I don't know when that might happen.

It all started like this: my initial listening showed the US4+ as being startlingly revealing to the signal chain. So much so that I decided to create a dedicated setup to quickly switch back and forth between three of my favorite DACs. Listening to the Niimbus seemed to give me just as much insight into my source as it did the amp itself, so I didn't want to rely solely on one DAC - however nice it might be.

At first, my simple plan involved multiple SOtM sMS-200 devices (I own a bunch), each driving a different DAC, feeding into the trio of analog inputs on the Niimbus. I could then switch back and forth on the fly.

The multiple SOtM solution didn't work out very well, for reasons I won't get into here. Instead, the chain ended up being more complex - Euphony PTS music server, USB out via BMC PureUSB1 active cable to a Matrix X-SPDIF 2 DDC, then AES/EBU linking the Matrix to a Titans Audio Lab Helen. The Helen (review forthcoming) is a fantasticbuffer/reclocker which has the added bonus of providing simultaneous outputs in AES, BNC, coaxial, optical, and HDMI I2S formats. This allowed me to feed several DACs at a time with impeccable signals. The Euphony server and Matrix DDC were powered by a Keces P8 linear power supply while the Helen used its stock PSU (which seems to give the best results).

On to the DACs. At first I went with a trio of modern favorites - Resonessence Labs Mirus Pro Signature, Wyred4Sound 10th Anniversary Limited Edition, and ModWright Oppo 205. Later I rotated through some classics - Assemblage DAC 3.1 Platinum, Sonic Frontiers SFD-2 MKIII, and a Stax DAC-Talent. I also used a pair of modern R-2R DACs at extreme ends of the price spectrum - the rather expensive Metrum Pavane Level III, and the budget Massdrop Airist RDAC. Factor in the miscellaneous other nice DACs on hand such as the Exogal Comet Plus, Cayin iDAC-6, iFi Pro iDSD, Yulong DA-9, RME ADI-2 DAC, and BMC UltraDAC, all of which had at least a bit of play time with the Niimbus.... and you can see how much listening was done.

Using any decent headphone allowed the Niimbus to very clearly delineate between these devices. Level matching was tricky and I confess to not being as precise when dealing with the vintage DACs. But in general, I felt like I learned to identify each distinct sound signature far better than I ever had before. I even gained new insight into gear I've logged thousands of hours on, such as the Assemblage. Some existing opinions were confirmed while others had to be reformulated based on this experience. Again, I may put down my thoughts on these DACs at some point, but the main focus here is on the Niimbus.

For headphones, I used a similarly absurd collection - Abyss Phi CC, Audeze LCD-4 and LCD-2, HiFiMAN HE6, HE1000 MKI, and Susvara, Sennheiser HD58X, HD650, and modified HD800, modified AKG K812, MrSpeakers Ether C, Ultrasone Edition 12, Focal Utopia and Elex, Sony Z1R, 64 Audio A18t, Empire Ears Zeus XR ADEL, UERM, Noble K10, and a bunch more that I won't bore you with. Most of these belong to me but a few, such as the Susvara and Abyss, were borrowed.

I also brought in various expensive amps to survey the competition - again, some owned and some borrowed. These included the Eddie Current Balancing Act, SimAudio 430HA, Eleven XI Audio Formula S, Auralic Taurus mkII, Woo Audio WA5LE, iFi Pro iCAN, Questyle CMA800R monoblocks, ALO Studio Six, Violectric V281, DNA Stratus, TTVJ Millet 307A (precursor to the Apex Pinnacle) and Pass Labs HPA-1. While not quite representing every single high-end amp on the market today, I feel this is an adequate sampling of end-game options - certainly enough to give the US4+ a thorough workout. Unfortunately I only had a few of these on hand at a time, so I don't feel qualified to fully document my thoughts on these models and how they relate to one another. I also messed up and forgot to document the tubes being used. The owners provided their favorite glass and I did not have time for tube rolling. This means the resulting comparisons must be taken with a grain of salt, as they only represent one possible take on the character of each tube amp.

For those who care about little details: rounding out the system was an Equi=Core 1800 balanced power conditioner, Cabledyne Silver Reference AC cables, and a variety of quality interconnects/digital/headphone cables from Cabledyne, BetterCables, C3 Audio, Effect Audio, Toxic Cables, and Moon Audio. Listening was done in balanced mode whenever possible. I do have a nice adapter from Toxic Cables, converting 4-pin XLR to 1/4" for those amps not equipped with balanced out.

I haven't done the math, but this is a cumulatively absurd collection of gear. I just wanted to point that out. Though the Niimbus itself has a substantial price tag, it's certainly not alone - plenty of these amps and DACs are priced in the same region or beyond.

I never planned for this to spiral out of control the way it did. Had I known from the start what this project would lead to, I would have carefully and methodically put each component through its paces, and chronicled everything in great detail. I also would have grabbed a bunch more pictures, even if they were ugly (this was a really messy few months for my system). That project would take an incredible amount of time, which I simply don't have. As it stands, I'll just be throwing out my somewhat disorganized thoughts, with snapshot comparisons, in hopes that they lead to a decent understanding of the Niimbus character. If you have a particular comparison you'd like to know about, feel free to ask, but I can't guarantee I'll have a good answer. If you don't feel like reading a novel (and I don't blame you), just skim to the section you want - the amplifier names are in bold for easy navigation.

Having said all that - the Niimbus US4+ absolutely belongs in this field of excellent amps. To my ears it handily outperforms most of the solid-state competition, and puts up a valiant fight against the best tube-based models out there. First, the solid-state options:

*The Auralic Taurus MKII ($1,900) sounds comparatively artificial and shouty, lacking subtlety and nuance throughout the spectrum. I don't at all find the Taurus overly bright in isolation, but that word appears several times in my listening notes when compared directly. There isn't a single headphone where I would choose Taurus over Niimbus.

Keep in mind, the Taurus is a very nice amp (Tyll and I both used it as a reference for some time), and for many people would represent a suitable end-game solution. Think of it as an "entry level" Porsche Boxster which is a thrilling vehicle yet can't keep up with the base model 911, much less the Turbo or GT3 variants.

*The Questyle CMA800R monoblocks ($4,000/pair) come off as comparably thin, lacking both bass depth and midrange body. Niimbus is significantly more palpable. Treble clarity, a major strength of the Questyle duo, is even better with the US4+, having a more natural, convincing feel. The only area where the monoblocks compete is the holographic soundstage. Driving an HD800 or K812, the Questyle amps do keep up with the Niimbus in that particular area. In many other aspects they feel a bit sonically compressed, for lack of a better term. Niimbus also takes a major lead when driving most planars, where the CMA800R combo feels comparatively underpowered and therefore dynamically underwhelming. Again, the Questyle offering is very competent in its own right, yet totally outperformed by the Niimbus (I'm going to stop saying this now, but keep it in mind for most of these amps).

I also found myself very much disliking the dual-mono nature of the CMA800R, with a separate volume knob for each channel. When the DAC has a quality onboard volume control then it's no big deal, but without that things get tricky. I guess I had forgotten how much of a hassle this can be.

*I had never heard the Eleven XI Audio Formula S ($5,500) prior to this project. The owner swears by it when paired with the Abyss Phi CC, which he also provided for this shootout (thanks again!). This particular example had the beefy (optional) matching PSU that seemed to weigh more than the actual amp itself. The amp with PSU is priced very closely to the Niimbus US4+, making it a natural competitor.

It's absolutely true that the Formula S excels with the Abyss; apparently the amp was designed with those specific headphones in mind. That particular combo is indeed breathtaking in several ways - particularly the crushing dynamics swings and bass performance which feels more like listening to speakers than headphones. Detail is nowhere near the same level, but with a good amp pairing I find it acceptable if not amazing. As most people know, fit is key with these headphones, and thankfully my big head seems like a good match. I remember having trouble when the original Abyss first launched but this time around things work very well - it still feels a bit weird, but isn't uncomfortable at all.

The Niimbus performs almost as well with Abyss Phi CC. It falls behind by just a hair in terms of layering - the difference is so small as to be hardly noticeable most of the time. Does that mean the Formula S is a better amp? Not really. The thing is, when I use Formula S with headphones from Focal or Sennheiser, it doesn't perform at that same level. It never sounds bad per se, but with most dynamic headphones I feel it falls short of sounding like a $5k amplifier. In fact it probably ranks down there with the Auralic for least impressive solid-state amp when using Utopia or HD800 or K812. Terrible? Not in the least. But it certainly doesn't sing the way it does with Abyss, and to a lesser extent, HE6 and Susvara. It just doesn't seem as refined as I would like - the midrange in particular sounds a bit dull, and transient response is not as snappy as I expect from an amp of this caliber. I don't exactly know why this would be. The Formula S puts out around 6 watts into 16 ohm loads, while the Niimbus does 7 watts into 50 ohms. I doubt it really matters either way - that's far more than needed for most any headphone.

Ultimately it seems the Formula S really excels with planar headphones. It does do quite well with certain dynamic headphones, like the Sony Z1R and Ultrasone Edition 12, but it mainly shines with difficult loads - Susvara, Abyss, HE6, and even the less difficult to drive LCD-4. Despite its prowess, I still end up preferring the Niimbus with everything but the Abyss. The Susvara in particular has more cleanly rendered transients and superior low-level detail when driven by the German amp. LCD-4 is a close call but tips slightly towards the Niimbus due to more nuanced bass texture, by about the same margin as the Formula S takes the prize with Abyss. HE6 is again a very close call but after careful listening I feel Niimbus has more believable treble.

In the end I would only recommend the Formula S over the US4+ if someone listened exclusively with the Phi CC and was certain they wouldn't stray from that product - which seems like a pretty bold decision. Though that combo issomething special, I still like variety in my headphone listening, and the Niimbus delivers that to a much greater degree.

*The Pass Labs HPA-1 is a superb amp. I'm not sure why it doesn't get more love at HeadFi. Perhaps its the lack of balanced drive which stops people from taking it more seriously? I don't know, but to my ears it remains among the absolute best amps in existence.

Niimbus gives it a run for its money though. The biggest strengths of the HPA-1 are its amazing treble clarity and wide open presentation. That's where it beats the Violectric V281, which is a touch more dynamic and punchy but not as refined. The Niimbus splits the difference - it's more dynamic than the Pass, but just as articulate and nuanced, if not even more so with certain headphones.

I'd call it a fairly even match with many headphones, but when you get into more difficult loads the Niimbus takes the lead. Susvara and Abyss don't quite hit their full stride with the Pass - they are very, very good, and I would certainly be happy with either.... until I switch to the Niimbus and discover both headphones have even more performance to be unleashed. In comparison the Pass is dynamically flatter and more "gray" sounding, as well as less tonally rich through the midrange. The same thing generally applies to the LCD-4 and HE1000, and even the Ether C to a smaller degree. All sound great with the HPA-1, yet all show improvement with the Niimbus.

With most dynamic headphones the differences are far less significant. HD800 does well with the added solidity of the US4+, whilst both Elex and Utopia seem to prefer the Pass for reasons I can't really put my finger on. Both amps do sensitive IEMs very well, which is rare even in this field of flagship amps - the 64 Audio A18t performs right up there with the absolute best full-size headphones when driven from either amp.

I do find the single-ended nature of the Pass to be somewhat limiting depending on the DAC involved. For example, the ModWright Oppo 205 sounds clearly better via XLR connection, as does the vintage Sonic Frontiers. In both cases, using the Pass Labs amp means "settling" a bit, rather than hearing the full potential of the source device. This may or may not matter depending on the DAC involved. On the flip side, the price difference is certainly something to consider, and the Pass amp is obviously more recognizable in both appearance and brand.

*The iFi Pro iCAN ($1799) is the least expensive amp used in this comparison. In some ways that shows - it's smaller, lighter, and less physically imposing than any of the others. But it's also a very enjoyable amp which earned high marks when I reviewed it at InnerFidelity a while back. It's got more options for sound adjustment/enhancement than anything else in this group, which makes for an interesting (and somewhat difficult) comparison.

Despite being very different in terms of features and focus, the Niimbus does remind me if the Pro iCAN in the sense that it is universally competent. While the iFi is sort of a "jack of all trades, master of none" in terms of the basic amp itself, I still find it very likable. It doesn't have any glaring errors in presentation. While not competing with the best amps out there, it falls short due to being merely very good rather than great in most areas. Other amps stumble by showing some obvious flaws which keep them in the lower tier, whilst the iFi does everything fairly well if not quite exceptionally. In that way it reminds me of the Niimbus, which seems to do everything superbly even if it gets bettered in a few specific areas by certain amps.

Of course, the comparison falls apart when you factor in the multitude of sound tweaking options on the Pro iCAN. If I say the US4+ sounds like an improved Pro iCAN, that's an almost meaningless statement - the question then becomes "what settings"? There are actually some instances where I can make the iFi my favorite amp in this whole group, when focusing on a specific DAC/headphone pairing and playing certain tracks. Again, that makes it nearly impossible to really compare, so I won't waste any more time on it.

*I don't really care for the SimAudio 430HA ($4,300 with built-in DAC module). In fact I once completed a lengthy and detailed review as to what I found lacking, only to have it rejected by my then-editor (not Tyll) as being too negative. I made numerous changes to soften the blow but apparently it still wasn't enough, so the review was canned (you can read the edited/nicer version here). My entire point was that the optional DAC sounded (surprisingly) great, the preamp functionality was awesome, but the headphone amp itself fell short. I stand by that assessment to this day, despite the disagreement of several friends.

The 430 can drive pretty much any headphone. It has a silent background for IEMs. It has an excellent volume control scheme which is very precise and feels great to use. And yet, the actual sound of the amp is just slow and uninvolving. Talk about a wet blanket all over my music. The Niimbus is so much more dynamically alive, with superior timbre, more convincing imaging, and a sense of "rightness" which I just find obvious. To my ears, there really is no comparison at all.

Interestingly enough, I actually think the more affordable SimAudio 230HAD sounds better in terms of amplification, and would steer folks in that direction if they simply must have a SimAudio product for their headphone needs. Despite my negativity, I've been a SimAudio fan for quite a few years now and would love to see them deliver a "proper" reference headphone amplifier. Ideally it would end up sounding a lot like the Niimbus US4+.

*The Violectric V281 ($2,939 with relay-based volume option) has long been one of my favorite solid-state headphone amps, tied with the Pass Labs HPA-1. It pairs amazingly well with every headphone I've ever thrown at it, with the caveat that it really needs balanced headphones to extract the full potential.

Since Niimbus shares some design aspects with the V281, you would not expect the general sound signature to deviate by much. And you'd be correct in that assumption. The basic presentation is definitely from the same family - robust, textured low end performance, weighty tonal balance, precise imaging, stunningly clear mids, and delicately balanced treble. Again, the V281 is a superb amp which is by no means hindered by the existence of the Niimbus line.

That said, I do hear some differences when switching back and forth. I'd start by saying the US4+ has bass performance both calmer and richer than the V281. A contradiction? Not really. It's effortless. Bottomless. Thunderous. Unflappable. All at the same time. Whether playing bombastic large scale orchestral works, Felix Hell pipe-organ, Gary Karr double-bass, or Edit-Select's Phlox, the Niimbus brings a sense of authority which V281 doesn't quite match. This can be heard on Keeno's Futurist, where the track "Bleary-Eyed" incorporates rapid thumping kick-drum strikes along with a simultaneous rolling bassline that really plumbs the depths. Niimbus handles both aspects with more control and resolution than any amp I've heard, including the V281 (which already performs at a very high level). It's really sort of breathtaking when the right headphones are involved.

The rest of the spectrum sounds more open and refined via Niimbus. It allows me to hear bigger jumps in quality when going from a good DAC to a great one, then all the way up to world-class level. It's also more likely to showcase the improvement brought by tweaks such as the Wyred4Sound Recovery reclocker (depending on the DAC, obviously). Again, V281 is exceptional in its own right, but the US4+ just scales higher, bringing with it new revelations into my playback chain.

Is this "in your face obvious"? Not always. When using the HE1000, Elex, Ether C, or LCD-4, the difference is fairly negligible. But with HD800 I notice it more. Susvara? Abyss? HE6? Yep, there's definitely a considerable improvement to be had. Interestingly, it doesn't seem to be merely a power difference. The easy-to-drive modded K812 really prefers Niimbus, as does the Sony Z1R, both seeming to appreciate the Niimbus' treble refinement and iron-grip more than its increased output. These are headphones I don't even like with certain amps, but here they sound very enjoyable.

Sensitive IEMs are another area where Niimbus outdoes the V281. Remember I mentioned the 10dB reduction in balanced mode? It's enough to make nearly any IEM present a silent background even via balanced output. I realize that most balanced IEMs use a 2.5mm connection or the newer 4.4mm style, but adapters for 4-pin XLR are easy to come by. Running a world class custom IEM like the 64 Audio A18t via Niimbus is really something special. The exacting nature of the volume control is very welcome, and the superbly linear presentation is just beautiful. Even still, some models are so sensitive as to still exhibit a bit of hiss - certain IEMs from Empire Ears, JH Audio, and particularly Campfire Audio seem bound to hiss a bit no matter what you plug them in to (but I'm sensitive to hiss anyway). In those cases single-ended mode may do the trick, with minimal sacrifice in SQ.

In the end, the Niimbus is either a minor step up over V281, or a large one, depending on the headphones involved. And the better your upstream gear, the more noticeable the improvement will be. Is it worth the money? There's no way I can answer that question for you. I'll say, without a hint of condescension, that for many people the Niimbus, V281, and even the V280 might be overkill anyway. One of the "lower end" Lake People amps may very well make more sense in terms of value per dollar. But for a statement-level experience, the Niimbus does deliver the goods across a wide variety of headphones. Make of that what you will.

That concludes the solid-state comparisons - now for the tube-based alternatives. As I said, these require a definite grain of salt due to my unfamiliarity with the tube configuration. While each amp had expensive glass which the owner had settled on after much trial and error, there's nothing to guarantee their favorite combo is the one I might like best. Indeed, many of these impressions conflict with what I've heard from these models in the past. Still, there is some value in the comparisons regardless. Note that I'm only listing stock prices here, knowing full well that at least a few hundred dollars will be required for quality tubes.

*First up is the ALO Studio Six ($4,100) which I believe came equipped with the upgraded teflon caps. Pricing after upgraded caps and tubes ends up being similar to the Niimbus. I'll be honest - I haven't historically liked this amp. I've heard it several times and thought it sounded slow, a bit veiled, and overly syrupy, yet lacking in the fun factor that sort of signature can bring when done right. This particular example admittedly sounded better, so either the cap upgrade or a wise choice in tubes (or perhaps both) made the difference. Still, I find it only marginally superior to the Cayin HA-1A MK2 which sells for $999. With the right tubes, I can come fairly close to replicating this type of sound with the Cayin, for roughly 1/4 the cost.

The Niimbus is quite a bit more neutral and resolving than the ALO. It portrays a sense of orchestral scale far more convincingly. It has superior bass texture, quicker transients, and significantly better treble clarity. About the only things the Studio Six does better is certain female voices - which can be rather beguiling with the right headphones - and a bit of euphonic treble coloration (not to be confused with treble control, where the Niimbus easily wins). I suspect many of the higher-end Audio Technica headphones would do great on the Studio Six - I'm thinking W1000Z, W3000ANV, and the W1000X in particular. I also think other potentially zingy headphones like the Fostex TH900 might be a good match. And I did have a good time using it with the stock HD800 as well as the Edition 12, both of which benefit from the darker/smoother presentation.

Still, I consider the Studio Six something of a novelty at best, and can't comfortably recommend it in this field of outstanding challengers. The Niimbus is easily superior to my ears.

*Woo Audio's WA5LE ($4,900) is an amp that I find very aesthetically pleasing. Just sitting there on my audio rack, I already have a strong appreciation for the device. Unfortunately the sonic performance does not live up to the amp's good looks.

This is very clearly a more colored presentation than the Niimbus. It has a lushness to the midrange, a sort of bloom or glow that is undeniably fun in its way. I can hear why some people fall in love with this amp. It brings a sense of fullness and body to headphones like HD800 which can lack a bit of soul from more neutral amps. Imaging accuracy is also excellent, despite the soundstage being only moderately large compared to the best amps in this shootout.

Bass control is lacking though, and treble is inoffensive at best, boring at worst. There were times when I felt resolution was quite good, but the very next track would show obscured micro-detail and underwhelming transients. I just couldn't get a consistent performance out of this thing.

Note that this was the more recent version with the "premium" upgrade from the factory. After tube upgrades, that probably makes it more expensive than the Niimbus. The Woo initially feels pretty dynamic but the somewhat gray background and undefined bass impact really put a damper on that in absolute terms. Again, I much prefer the Niimbus for all but a few very specific configurations, and even then I think you can do better with one of the other tube-based offerings in this roundup. I do seem to recall hearing much better sound out of a WA5 several years back, so this may just be a poor choice of tubes for my preferences.

*The DNA Stratus ($3,000) is also quite attractive in its own unique way, like a slab of cotton candy with tubes and transformers sprouting from the top. It doesn't match a single thing in my audio rack, and that's just fine.

To me, this amp really captures the traditional "tube sound" in a positive way. It's got excellent detail, a very black background, and somehow manages to tame treble harshness while remaining highly articulate - the balance in the treble region is very well done. The weak spot seems to be low-end performance, where it is good but not great.

Niimbus is comparatively more dynamic, with an even blacker background (though again, the Stratus is already excellent). The solid-state amp comes across as more linear, more even handed, but also more punchy and bombastic. And that bass? Control, texture, extension, all go to the Niimbus by a large margin. Treble on the Niimbus is a touch more prominent, doing less favors when playing poorly done material but exhibiting more clarity when playing quality tracks. I also note the solid-state amp successfully drives a wider range of headphones, from sensitive IEMs to difficult planars and everything in between. Stratus does well enough but isn't the greatest at these sensitivity extremes.

Still, there's something magical about the sound of the Stratus. I could absolutely see headphone nuts owning both amps, alternating between the head (Niimbus) and the heart (Stratus) as the mood strikes. I have yet to hear the newer/more expensive Stellaris, but if it's anything like the Stratus, I'm already sold on the sound (if not the price).

*The TTVJ 307A ($6,000) is a somewhat rare amp designed by the legendary Pete Millett. It launched about 10 years ago in limited quantities, and is the precursor to the (slightly) more well-known Apex Pinnacle.

Of all the tube amps in this comparison, the 307A sounds most similar to the Niimbus US4+. Both models are largely neutral, with similar tonal richness and excellent speed. Both have superb micro-detail and a correspondingly well-defined soundstage. The main difference seems to be at frequency extremes, where the 307A manages a silkier treble and the Niimbus has more low-end solidity. At first glance, this makes the Niimbus feel darker/more bass oriented, and the 307A feel lighter/brighter. But this is an illusion of sorts, and more focused listening reveals the amps to be much more similar than different.

The 307A is a potent amp that actually does quite well with difficult loads. Yet Niimbus is even more versatile. Something like an HE-6 or Susvara played at really high volume has the 307A losing just a bit of composure, while the Niimbus feels effortless. And with more sensitive headphones like the Utopia or K812, the 307A comes on a bit strong at low levels - you really don't get much usable travel on that volume knob. Niimbus, with its adjustable pre-gain and 256-step attenuator, offers much more range and thus a more enjoyable experience.

There doesn't seem to be many 307A amps in the wild. So this comparison perhaps isn't very useful. But for those who have had the pleasure of hearing this fine amp, be aware that the Niimbus US4+ has quite a few similarities. It's not an exact match, but comes surprisingly close considering the variance in topologies.

*Eddie Current's Balancing Act ($3,950) is one of my favorite amps ever. The version used for this comparison used some extremely costly tubes though again, I didn't write down the specifics. I believe this one had the Electra-Print transformers but I could be wrong on that (I had not realized how many variations of this amp existed, or I would have paid more attention). The resulting sound was stunning - liquidity, air, finesse, and just a totally organic experience that made me want to forget all about the evaluation.
Hours later, when I snapped out of my daze, I was able to spot some things that I liked, and a few which I didn't, in relation to the Niimbus. On the positive side, the Balancing Act has a more credible sense of air on the top end. It outperforms the Niimbus and even the 307A in this regard. Cymbals are spectacular on both of those devices but the Balancing Act is about as close to perfection as I've heard. The other thing Eddie Current's machine gets right more than perhaps any other amp is the sense of flow. It's a transient response thing, where sound is simultaneously quick and yet relaxed... notes explode forth with great intensity and speed, but also convey a sense of ease which is difficult to explain. The resulting sound is very fluid and articulate whilst avoiding any sense of artificiality or fatigue. That probably explains why I didn't want to stop listening.

On the flip side, the Niimbus US4+ has several areas where it outperforms the Balancing Act. For one, Niimbus has an absolutely jet black background. I had no complaints about the BA until I listened back to back and heard how inky the Niimbus is in comparison. If the Balancing Act is a really nice older Plasma display (think Pioneer Kuro or Panasonic ZT60), then the Niimbus is an OLED.... the one has excellent blacks and initially seems tough to beat, but then you realize other is very clearly superior. This, combined with the explosive dynamics and clearly articulated bass textures make the Niimbus feel more "full-range" for lack of a better word.

I also note the Niimbus is more comfortable driving a wide variety of headphones. When the Balancing Act finds a good dance partner - HD800 and Utopia being my favorites, with Elex and K812 not far behind - it sounds magical. But when tasked with a difficult load such as Abyss or Susvara, it simply runs out of steam. Even moderately difficult planars such as LCD-2 and HE1000 can sound a bit uninvolving out of the BA, particularly when playing quiet jazz or classical recordings. Meanwhile Niimbus drives all of these (and more) to their fullest potential without breaking a sweat.

It looks like the Balancing Act is now discontinued. So again, perhaps not a terribly relevant comparison. Nonetheless, I consider this a benchmark amplifier if ever there was one, and I know multiple folks who continue to use it as a reference. Judging by this particular example, I wouldn't say the Niimbus totally blows it away by any means, but the solid-state options ends up being my preference more often than not, and with all but a few headphones.

So there you have it - countless hours spent listening, not to mention swapping configurations. Dozens of headphones, a collection of DACs worth more than my car (which isn't really saying much), and at least a dozen top-class headphone amplifiers. After all that, does the Niimbus Audio US4+ smoke them all?

Hardly. That sort of overly-simplistic comparison doesn't reflect the complex realities of comparing excellent gear. Most of the amps in this roundup have strong points where they lead the pack. None of them does everything the best. Which means choosing comes down to preferences and priorities.

The high points of the Niimbus amp are plentiful: strikingly silent background, robust power output, and a wonderfully balanced signature with excellent detail retrieval and superb tonal accuracy. Then there's the finesse of that unique volume control solution which just works like a dream - perfect level matching at all volumes, plenty of steps for fine tuning, and quiet operation (unlike its predecessor). It's a joy to operate, and frankly makes most other amps feel underwhelming.

If I'm wearing my metaphorical audio reviewer hat, the Niimbus is probably my number one pick overall. It has the greatest "headphone bandwidth", meaning it works exceedingly well with every single headphone I throw at it. Other amps may have greater synergy with a specific headphone or two, but as a whole, the Niimbus is the most versatile amp here (with the possible exception of the Simaudio, which is similarly versatile but just doesn't sound very good to me). In addition, the Niimbus has among the best clarity and insight of this entire group. It allows me to hear the character of each source to a degree no other amp can quite match, though a few come close. When evaluating a new DAC or player, the US4+ would definitely be my tool of choice.

Areas where the amp doesn't shine as brightly include the somewhat polarizing appearance (which, again, isn't quite finalized, but I doubt will stray too far from the prototype), the high price, and a relative lack of emotion - chiefly compared to some of the tube-based competitors.

That means if I switch gears to "hobbyist/music lover" mode, a good case could be made for choosing any number of worthy contenders over the Niimbus.

In tube land, the Balancing Act might tempt me away with its stunning treble, organic flow, and retro good looks. It's been discontinued but used examples aren't too hard to come by. The 307A is more of a rare beast but might be worth tracking down - it's similar to the Niimbus in many ways, but has a slightly more ethereal presentation which is, at times, preferable. I might (strong emphasis on that word) actually like the 307A better than the newer, more expensive Apex Pinnacle that superseded it. Lastly, the DNA Stratus offers a pleasingly stereotypical "tube sound" that I could easily fall in love with - at a significantly lower price than Niimbus.

Speaking of lower prices - in the solid-state world, the Pass labs HPA-1 is a tough competitor, and hobbyists certainly must take value into consideration. True, the Pass lacks some features like balanced mode, and doesn't do as well with really difficult planars. But aside from that it is nearly the equal of the US4+ in many ways, for substantially less money. It also gets bonus points for being far more available when it comes to listening before you buy, as Pass Labs probably has better distribution than anything else here in terms of brick and mortar shops. Beyond that, the Violectric V281 remains a very relevant option. Yes, the Niimbus is better in several key areas, but the price differential may be larger than the performance delta. Unless the user owns an absolutely top-notch source (or plans to acquire one in the near future), I would hesitate to write off the V281.

Does anyone really need to spend $3,000-$6,000 on a headphone amp? Probably not. Spending hundreds rather than thousands is clearly the more reasonable approach, and can still get you a very solid result - see various Violectric/Lake People models, as well as the Arcam rHead, Rupert Neve RNHP, etc. But for those chasing the "ultimate" experience, Niimbus Audio offers a worthy contender with a unique skill set. After extensive listening, I put it in the running for best amp out of the dozen models at my disposal, and certainly the most useful as a reviewing tool. I realize the price is very high, and obviously don't recommend it for everyone... but for certain people it is a compelling option worthy of consideration.
Thank you! I don't have the energy to mess with my own site... I get paid for my submissions to Darko.Audio and InnerFidelity, and that's enough for me. Plus I always enjoy contributing to HeadFi just for fun. No plans to change my situation anytime soon but thanks for the vote of confidence!
Thanks for the review, John. I usually prefer a warmer sound from SS. In this case, do you think I'd prefer the V281 over the Nimbus? Thanks.
I wouldn't necessarily call the V281 warmer - the tuning is very similar between both amps. But I suppose the Niimbus has such clear treble that it can sort of steal the spotlight a bit, giving the perception of a "brighter" sound at first listen.