Review: Niimbus Audio – US4+

Niimbus US4+



In this review the Niimbus US4+ was mostly used with the Violectric V850 DAC, as well as with the Atlas DAC. We won’t be analyzing the Line Out feature in this review but I can and will say that I have been using it at home as a kind of pre-amp in my speaker setup with a DENON (integrated) poweramp. I really improves the dynamics, clarity and tightness and I have seen other users on Head-Fi report he exact same thing.

You might have read it elsewhere but there indeed is the tiniest bit of noise audible in the form of clicks when turning the volume control. This is very minimal though and it isn’t annoying to me at all as I don’t change the volume that much.

For the rest he US4+ has a perfect pitch back background and great clarity and cleanness. Niimbus worked hard to get the noise level as low as possible and they have successfully used several technical tricks to do exactly that. The US4+ to me performs “better” compared to the V281 in all ways, but it’s especially this clarity and clean sound which impresses when switching to the US4+. The Niimbus also sounds tight and fast with a great prat and the US4+ is always in control, no matter if it’s with deep bass or high treble. The general tuning is balanced and linear but now more to the neutral side. The US4+ performs how a high end unit should perform, and Niimbus chose to have neutral, detailed tuning with a musical, smooth, and very slight warmth in the delivery. You get the best of both worlds this case, unless you’re looking for a dryer, more analytic and fully neutral delivery.

The sound stage is wide and deep but to me it’s wider than it is deep. The layering is good but at the same time not as pronounced. The separation and spaciousness are excellent and on level of what you’d expect from a unit at his level. The US4+ excels in dynamics and detail retrieval and you’ll always have the best texture, no matter what you’re listening to.

I read a post somewhere which stated the US4+ was a bit emotionless, but I really can’t agree with that. With the US4+ it’s important to use a good DAC and of course if you choose a dry and flat one there, the end result might not have the best emotion levels, but I have to say that in my test systems, I didn’t ever get this impression. Sure it’s not tube lush, but I don’t think that’s Niimbus goal either.


My readers have been asking me to include more full sized headphones in the reviews and on HFN in general, and this is the excellent occasion to do this as the US4+ can drive them all.

Therefore I’ll be describing the synergy with a bunch of flagship headphones both in single ended as well as in balanced mode. Unfortunately the Empyrean is missing in this list as it’s getting a connector repaired, it happens.

Hifiman HE-1000SE

My favorite HE-1000 of the series, though I really like the original one as well. The SE combines best of the earlier models and the synergy with the US4+ is excellent, especially in balanced mode.

In balanced mode you get a fuller sounding HE-1000 with thicker notes and more impact in all regions. The midrange spaciousness and overall sound stage width also is best in this mode. Bass is full, goes deep when needed and has good layering. The US4+ is always in control and the bass is tight, fast and punchy at all time. The mids are musical with the best texture. Vocals are perfectly integrated, sometimes a little more forward placed but in general they sound natural and realistic. The upper mids and treble region are energetic, well layered and extended. Control, speed and energy are 3 key words for this combo.

In single ended mode the playback volume is lower and you get less body, impact and energy. Bass is light yet connects perfectly to the lighter mids. It’s mostly body and impact which are light here buy everything is linear and balanced. In single ended mode you get a flatter, more neutral sound signature with this Hifiman. My personal favorite is clearly the balanced mode.

Focal Utopia

The Utopia is a headphone I personally really like and I bought it just to have it in my collection, and not to specifically review it.

The Utopia is famous for its speed, micro details and impact and you get a really excellent sounding Utopia in balanced mode. Just like with the HE-1000SE you get a fuller sound that has bigger impact and body in bass, mids and treble. Vocals are again a little more to the front, more so than with the HE-1000SE.

Bass has body and a lot of impact in balanced mode, but it’s also tight and fast and the US4+ is perfectly in control. The mids are very spacious, with great layering and extension, the decay is exemplary. Treble is dynamic, energetic and extended.

In single ended mode the Utopia sounds lighter and more linear with less bass impact and overall better linearity and balance. The sound signature again is more neutral in single ended mode and the impact and body are more civilized. For some the balanced signature/synergy might be a bit much body wise, and those listeners for sure will prefer the single ended mode. I can appreciate both the balanced as well as the single ended output, and the Utopia does sound better in SE mode than the HE-1000SE does.

Sennheiser HD800

Amplifier synergy is critical when it comes to the HD800, and I myself prefer listening to it with a warm solid state amplifier or a smooth tube amp with a good kick.

The Niimbus US4+ of course isn’t either of those but that doesn’t mean the combo doesn’t work. In single ended mode you get a really nice sounding HD800 which is linear, balanced, fast and precise. Body over all isn’t the biggest but it’s just right for a neutral headphone. The vocals, sound stage and extension in this combo is extremely good, but that was to be expected. The mids have incredible texture and are pure joy to listen to. Treble is extended and detailed and comes with great energy but it’s never harsh or sibilant.

In balanced mode for me the signature is a little too v-shaped with bigger bass and very energetic treble. This might be something you like and it’s probably good for Dance, Pop or R&B but then the HD800 probably isn’t the headphone you’re using. For me the HD800 sounds best in single ended mode. I have to admit that I prefer listening to the HD800 with the warmer Violectric V281, or even better a warm tube amp such as the Lafigaro 339 or the Auris Audio HA2-SE.

Hifiman HE-6SE

Even though it’s the SE version, the HE6 still loves and needs power and the US4+’s volume control goes up quite a bit, yet it’s still perfectly doable without extra gain. You might prefer it with a higher gain level, but that’s a personal choice.

In balanced mode the HE-6SE get good bass body and impact but all this in a civilized, tight and fast way. Bass has excellent layering and texture, and it has a really nice kick to it. Bass goes really low when needed but doesn’t exaggerate end it perfectly connects to the mids. These mids are spacious, precise and have really lovely texture, extension and decay. Layering and dynamics everywhere are really good and the voices sound soft and very textured. Treble again isn’t sibilant but energetic, dynamic and precise.

In Single Ended mode the HS-6SE gets less body and sounds flatter, more neutral. The difference is quite remarkable. The HE-6SE still sounds incredibly good, just lighter and flatter and it doesn’t always make the above mentioned characteristics as easy noticeable, but they’re there.

The HE-6(SE) always has been a headphone that requires a good amp, but the Niimbus US4+ has no problems making it sound good.

More headphones and a comparison vs the V281 and much more, all on the last page


Lieven is living in Europe and he's the leader of the gang. He's running Headfonia as a side project next to his full time day job in Digital Marketing & Consultancy. He's a big fan of tube amps and custom inear monitors and has published hundreds of product reviews over the years.


  • Reply June 1, 2019


    i dont think v850 would make jusctice for amp like this. This dac i below average. We get it youre fanboy of violectric but atleast try other dacs maybe? im sure Chord qutest would do much better let alone costlier dacs. Good read though

  • Reply June 7, 2019


    v850 is not a bad DAC. It does about everything right except the detail retrieval compared to others in the price range. For a detail freak like myself, it’s not an ideal DAC. But, it can be a great DAC for music lovers who focus more on the timbre, naturalness, etc. But I do agree Chord qutest is better, and I suppose that’s about the minium DAC people, who spend 5k on a headphone amp, will use together.

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