REVIEW - NUFORCE PRIMO 8 - AN AUDIOPHILE PHASE COHERENT QUAD BALANCED ARMATURE DRIVER IEM
My first venture into the world of IEM was with the Shure E2c in 2007, when I wanted some isolation for private listening at home in bed. Prior to wanting the extra isolation I used some Bose noise-canceling headphones, because they helped remove the noise of my window air-conditioning unit. But I was mostly into using speakers, and felt that the entry level E2c didn't have the clarity or frequency balance of my vintage Polk Compact Reference SDA speakers. Those speakers are still amazing sounding today, despite the treble fading a few dB as the crossovers age and show signs of needing to be refurbished.
Within 6 months of getting the E2c I began to wish for an affordable upgrade that would give me more of an "audiophile" level of performance. I tried the jump to the Shure E4c and the detail and sonic balance was much improved, at the expense of having a less energetic and efficient performance, and worse bass impact. My iPod just didn't drive them very well. I then bought some $200 Denon IEM with dynamic drivers that had much improved efficiency and bass impact, at the expense of worse sonic balance and less accurate timbre and tone. At low volumes the sound was fairly enjoyable, with a "V-shaped" frequency response, but the sound would become fatiguing at higher volumes. That brought me to head-fi where I started looking for portable headphone amplifiers to improve the impact of my E4c without losing the sound quality.
After my introduction to Head-Fi it seemed like I began an almost endless journey through every popular IEM and portable amplifier, trying to see just how good audio reproduction in such a tiny package could get. During that time several IEM stood out well above the crowd for me, starting with the Westone 3 in 2008 when used with the proper tips for my particular ears. With silicone tips the treble could get a little boosted, and with triple flange tips the treble could sound a little artificial, but with the Complys foam tips the treble was sweet and smooth, with strong powerful bass and detailed mids. With Westone UM-56 custom tips the midrange was more present and vibrant, with a good balance between bass, mids and treble. To this day it is hard to fault the W3 with Complys foam tips or UM-56 custom tips.
Subsequently the Westone UM3X caught my ears, and offered a more audiophile frequency response while working with a wider variety of tips and ear canal shapes. But the UM3X had slightly less midrange warmth and a bit less depth of soundstage than what the W3 offered. Still, performance was on a comparable level, depending on your tastes, with the W3 being slightly recessed in the mids and the UM3X being slightly forward in the mids. Both IEM perform well un-amped, and scale up as the source and amplifier are upgraded. My main complaint with the UM3X was that they could sometimes sound a little congested in the mids with complex musical passages.
Then the Westone 4 came out, which I had the pleasure of secretly beta testing early versions through production, and they corrected most of the issues with the W3 and UM3X, which I still own and love. They brought back the larger soundstage of the W3, with the balanced bass of the UM3X, and added just the right amount of midrange presence. The problem with the W4 is that they are more laid back in the highs and less energetic in general than the others, and sometimes they need an amplifier to wake up a little better. The W4 did need a redesign of the silicone tips to sound more open and detailed, which happened after a couple of years, and the new tips took the W4 up a notch in performance (sounding slightly veiled like the HD650 with the original tips, and more crisp like the HD600 with the new tips - the comparison between Audeze LCD-2 rev1 vs rev2 could also fit here).
At that point I stopped looking at high-end universal fit IEM, because between the W3, UM3X, and W4 I owned three top-tier IEM that could be rotated through depending on the source, amp, and program material I wanted to listen to. Plus I had many flagship custom IEM that I would use for more serious listening. I mostly use the universal fit IEM when I am out and about, so I worry less about damage or loss because they cost less than my custom IEM.
I did get a pair of HiFiMan RE-600 as a gift, and when well amplified by the HiFiMan HM-901 with balanced amplifier, they sound very much like a well amplified Westone 4, with slightly smoother treble but otherwise similar timbre and tone. Basically I mostly enjoy the RE-600 with my HM-901, which I will occasionally use at my bedside table; but I don't feel that my iPhone or iPad properly drive the RE-600, and so they also get limited use.
I still own many other nice high-end IEM that I have enjoyed and didn't want to part with, but they rarely get any listening time these days, including the UE Triple.fi 10 Pro, Shure SE-530, Image X10, Westone 2, Monster Turbine Pro Gold, and HiFIMan RE-262. Each of those has something I'd want to fix - more mids in the TF10Pro or W3, more detail in the W4 and MTPG, more bass in the W2, More treble in the SE-530 and Monster, more treble and bass in the RE-262, more efficiency in the RE-600, and less upper-bass bled into the mids with the X10.
So, because of my large experience with universal IEM in the past, and also with 7-8 different custom IEM ranging from low-end to flagship, I was asked last November to beta test Nuforce's new Primo 8 IEM. I've worked with Nuforce to beta test other products, and they have always been very responsive to suggestions and the final versions have always been improved and something that I would want to own.
I have heard 4 different versions of the Primo 8, including the final production version, and must say that Nuforce has done a great job bringing out an audiophile quality balanced armature universal fit IEM.
SPECIFICATIONS & FEATURES FROM NUFORCE:
Model Name: NuForce Primo 8 - MSRP $499
Full Product Name (for online store): NuForce Primo 8 Phase-Coherent Quad-Speaker Earphones
Slogan: Your Personal In-Ear High End Speaker System
When the goal is to replicate the sound quality of reference-class, multi-driver high-end speakers in your ear, the NuForce Primo No. 8 earphone stands well apart from the crowd. To achieve this elusive goal of breathtaking realism, explosive dynamic and natural sound, the Primo No. 8 embraces patent-pending technology to capture the best of both worlds: the coherence and seamlessness of a one-driver earphone, with the detail and transient speed of a balanced armature speaker array.
Balanced Armature Drivers:
Balance armature drivers deliver the highest efficiency of any in-ear speaker design available. NuForce has fitted the Primo No. 8 earpiece with four proprietary balanced armature speakers to form a unique 3-way design. Two units operate as one for bass frequencies, with one speaker each for the midrange and treble bands. With two speakers working in tandem to reproduce the lowest frequency ranges, the listener experiences the purest, most natural bass possible - in effect, doubling up on bass speakers reduces overall harmonic distortion by over 75 percent. The Primo No. 8’s swift, rock-solid bass operates as the foundation for dynamics heretofore available only in the most costly over-ear dynamic and electrostatic designs.
Music lovers who have used balanced armature earphones will discover that the Primo No. 8 is far more refined and coherent than anything that they have ever experienced before. The Primo No. 8’s remarkable midrange resolution, clarity and neutrality are among its most distinctive features.
Patent-Pending Linear-Phase Crossover:
The graininess of other balanced armature earphone designs is often the result of crosstalk between drivers due to their crossover network’s inability to prevent overlapping. That is the achilles’ heel that plagues all other balanced armature earphones until now.
In contrast, Primo No. 8's four-driver speaker array employs a crossover network that seamlessly distributes music’s frequency bands utilizing a proprietary first-order Butterworth filter design.
First-order Butterworth filter designs are not without its problems. It offers an often insurmountable challenge for home stereo speakers owing to the required power levels along with difficulties relating to off-axis performance and room interactions. As a consequence of an earphone’s far smaller size and implementation, these issues simply don’t apply. However, an obstacle that does stand in the way of the Butterworth filter used in earphones is, again, size – a lack of space for the crossover’s parts. Here’s where the Primo No. 8 shines. NuForce has addressed the size issue by developing a proprietary first-order Butterworth crossover network requiring fewer and smaller parts than ever offered before. The Primo No. 8’s proprietary crossover design has the earphone’s four speakers singing as one.
While the technology is far too complex to summarize here, the results are abundantly audible. Transient speed is incredible. Plucked strings take on an amazingly life-like presence. Vocalists sound as if they are there in the flesh. Percussion explodes in a truly three-dimensional sound field. The subtlest of details appear magical and the sound never fatigues. The audiophile can now enjoy the promise of a true linear-phase crossover design absent the drawbacks plaguing products that have employed these topologies in the past.
Proprietary Cable Design:
To achieve the Primo No. 8's unprecedented sound, nothing less than the best audio cable will do. There is more to the apparently smooth and lightweight cable than meets the eye. Inside this unique NuForce star-pattern Litz cable are several advanced technologies to achieve the most accurate sound reproduction possible.
For strain relief, typically a weak point of earphones, the Primo No. 8’s cable is built around a core of Kevlar Silk. This core is then surrounded by a star pattern of seven silver wires. A layer of insulation envelops the silver wires, which in turn is surrounded by another star-pattern of nine bundles of fine gauge, seven-stranded, individually insulated OFC wires – a technique called Litz construction. Litz cables, most often encountered in radio-frequency transmission technology, are especially effective in the delivery of high frequency signals. The entire cable assembly is jacketed in a pliable polymer to eliminate mechanical noise contamination. Taking cable design to its zenith, the Primo No. 8 employs isolated left and right ground wiring to reduce inter-channel crosstalk and maximize channel separation.
Maximum Noise Isolation:
A reference-level earphone requires ear tips that provide a good, solid seal, particularly important for good bass performance and privacy. Toward this end, the Primo No. 8 employs maximally effective Comply Isolation Foam tips.
The Final Word:
In capturing the best of both worlds – the coherence and seamlessness of a one-driver earphone, with the detail and transient speed only a balanced armature array can offer – the Primo No. 8’s innovation, performance and value are nothing short of unique. Having experienced the Primo No. 8, the listener will see that no detail has been overlooked. The NuForce Promo No. 8 earphones deliver the most natural sound that audio technology is capable of providing.
Included with the Primo 8 are:
- Four (4) sizes (two pairs per size) of silicone ear tips
- Two (2) pairs of Comply Isolation foam ear tips
- A designer carrying case
- Two (2) sets of cables (with and without microphone)
- Cleaning cloth
- Cleaning tool
- 3.5mm to 6.5mm adapter
- In-Ear Audiophile-Quality Speaker System: Four balanced-armature drivers in each earphone deliver a smooth, extended response, explosive dynamic realism and fatigue-free sound.
- Ideal Balance and Accuracy: A patent-pending three-way phase-coherent crossover design achieves perfect linear phase performance resulting in a natural, fully dimensional soundstage, lightning-fast transients and crystalline transparency.
- Breathtaking Realism: A captivating midrange and delicate highs envelop the listener in musical textures almost real enough to touch.
15" Macbook Pro Retina as transport, with CEntrance HiFi-M8 and DACport DAC/amp, Audioengine D3 DAC/amp, or iPhone 5s and iPad Air headphone out and with Pico DAC/Pico Slim amp via CCK.
IMPRESSIONS - HANDS ON:
Before I comment on the sound, I want to note that the combination of Complys foam tips, over-the-ear guides, and the rotating coaxial cable connector make it harder to get these earphones inserted without the cable getting in the way.
The weight of the ear guide would sometimes cause the cable to rotate in the socket, and that often puts the guide and cable right between my ear and the ear phone, blocking it from being inserted. This usually happens because I'm rolling up and compressing the foam tip and then keeping it compressed with two fingers while inserting it with one hand, while the other hand is pulling the ear lobe back to open up the ear canal better - and so there is no extra hand left to control the cable when it decides to rotate under it's own weight.
My pre-production Primo 8 came with only the foam tips, but when I switched to some Westone silicone tips (old style) it was much easier to insert the IEM one handed without issues, because I don't have to use my other hand to pull my ear lobe back to open the ear canal more. So I can use my other hand to keep the cable from rotating out of place during insertion.
I mentioned this to Nuforce, and suggested that a removable guide like with the Sennheiser IE8 might make more sense, or a tighter coax jack that doesn't let the cable rotate as easily. That was when I found out that the Primo 8 will be shipping with a small notch around part of the edge of the cable socket, with a small bump on the end of the cable's plug, which will engage with the notch and will lessen the chance that the cable will rotate more than 20 degrees total. You can intentionally force it to rotate farther if you want, as the plastic bump is flexible and can deform under enough pressure to rotate the cable.
Once the IEM are in place, with either foam tips or silicon tips, they are very comfortable to leave in for hours. I also very much like the 3.5mm plug which works with the small hole in my Apple Brand iPhone case, while others like my JH Audio custom IEM cable won't fit through the hole in my case.
We discussed the possibility of a cable with microphone, and it looks like that option may become a reality. The problem is that the coaxial copper/silver combination Litz cable is actually fairly costly, as they ship the Primo 8 with what is already the equivalent of an upgraded aftermarket cable.
Since these are balanced armature IEM they should not need a lot of burn-in, and with the most recent pair, which had no hours on them before I got them, I didn't hear much of a change in the sound outside of the first 24 hours - they opened up slightly in the first 24 hours with no change in sound signature. It was immediately obvious that the Primo 8 was a very detailed and well balanced sounding IEM.
I did most of my early listening with the Complys black foam tips that came with them (Westone compatible bores), and later as I wrote this review I re-listened to most of the music with some old style Westone grey silicone tips. I am expecting to get some Nuforce silicone tips to try later, but until then I can say they sound great with the old Westone tips which are readily available. The sound is slightly more detailed and open with the Westone silicone tips which I preferred, but the foam tips do an excellent job as well. Isolation was good with either tip.
I first tried them using the iPhone 5s and CEntrance HiFi-M8 digital DAC/amp, and I tried a wide variety of my favorite lossless Jazz and New Age music, and then listened to some Blake Shelton country music, Maroon 5 rock music, and some Christina Aguilera pop music. I moved on to try at least 50 different artists in a wide variety of genres, with 90% of the music in Apple lossless format as high as 24/192 bit rate. I then tried other sources like iPhone and iPad headphone out, and other DAC/amps.
Unlike the earlier prototypes where I was often wishing for more bass impact, the final version has just enough bass to satisfy most people including me. The Primo 8 are not bass-lite or cold sounding in any way, but they are also not a bass-head's dream. The bass response is what I would call neutral.
It's there and present, and it's very fast and detailed, as well as very nicely extended to below 20Hz with test tones. Bass texture is excellent (timbre & tone). String bass plucks sound crisp and speedy, with the low notes keeping up with the upper harmonics. Bass drums have that nice deep bass ring, just with slightly less impact than my W4 or UM3X. I do think that the bass impact was slightly improved with the silicone tips vs the Complys foam tips.
I switched the HiFi-M8 DAC/amp output impedance from 1 ohm to 10 ohm and the low-end impact improved a little more, so I left it there. It wasn't necessary to use the bass boost switch, although the Primo 8 were able to handle the bass boost very well without being overdriven at high volumes. My DACport also has a 10 ohm output impedance, and the Primo 8 sounded good with that as well.
I also tried the iPhone 5s headphone jack, and thought this had a little better bass impact than the HiFi-M8 at 1 ohm output, although the 5s is not as clear and spacious sounding in return. So source and amp choice will make a difference, and the Primo 8 scale up very nicely but don't require an amp. **
** Note - The iPhone 5s headphone jack also sounds a little more forward in the mids when compared to the HIFi-M8 or Pico DAC/Slim with a CCK, and you definitely benefit from adding a DAC/amp to the iPhone 5s when trying to wring out the most from any IEM. Changing to the Macbook with most any DAC/amp confirmed that the iPhone 5s headphone jack is lacking the detail, freq balance, and space of a nice DAC/amp. If listening un-amped I would prefer to use my iPad Air which sounds smoother and more detailed than the iPhone 5s.
Anyway, I don't consider myself an exclusive bass-head, as I can enjoy neutral and spacious sounding headphones like Sennheiser HD800 and Stax SR-009. Some would say that these two headphones are bass-lite, but when the SR-009 are driven by a Blue Hawaii amp or the HD800 are driven by an EF-6 amp then they cannot be called bass-lite.
Yes, headphones with really strong bass are a lot of fun, but it can be a problem if the headphones add more bass than what the recording calls for at the time. The HD600, HE-500, or LCD-2 rev2 are other example of headphones with a good balance between bass, mids, and treble, and they are less picky about what amp you pair them with to get strong bass out of them (vs the HD800 or SR-009).
So I played some music looking for ways to characterize the Primo 8 vs those two full-size flagship headphones, and one particular track was Ryan Adams "Dirty Rain" where it opens up with an acoustic guitar and then after a few seconds the bass guitar comes in, and after that the drums rise to a slow beat. I was able to place the singer at front center stage with the drums just behind him, with the acoustic guitar off to the left and slightly behind, and the bass player slightly to the right, with the piano farther back and more to the right.
The bass guitar and bass drum were present and accounted for, and although they wont make your ears rattle they did offer a deep bass foundation. In fact, all the different instruments and vocals seemed to be at similar levels to where I expected them to be, having listened to the same songs with the HD800 or SR-009. Some IEM like the V-MODA Vibratto can exaggerate the bass too much, and generally sound less accurate or muddier, with recessed mids in relation to their bass. That puts the singer further back, and almost on top of the bass drum behind him.
I also could not place the location of the bass guitar with the V-MODA at all. It was like having a "bass cloud" surrounding me with the V-MODA. The Westone 3 also made the bass player a little more difficult to locate than with the Primo 8, although the bass speed/detail was much closer to the Primo 8 and a big step up from the V-MODA.
I switched to electronic music with the Primo 8, including Infected Mushroom "Dancing with the Kadafi", and the music surrounded me from all directions during the intro, and then when the bass synthesizer kicks in around 1:20 I could feel the pulsing rhythm with decent impact (same thing with some Lindsey Stirling electronic music, and others). No, these wont hit you like a V-MODA, Westone 3, UE11Pro, or JH16Pro, but there really isn't anything really missing down there either.
Would I say that the V-MODA or Westone 3 are a little more fun with bass heavy electronic music like Infected Mushroom? Sure, but when it comes to actual acoustic instruments and vocals the Primo 8 leaps ahead. The Primo 8 did sound punchier with electronic music as volume levels went up, but then that could become harmful to your hearing with longer listening sessions. Speaking of volume, I'm finding that they are a little more efficient than my Westone 4 as well.
In general, the timbre and tone of the Primo 8's midrange is spot on. The Primo 8 has a slightly more forward or slightly more aggressive midrange in comparison to the W4, so loud listening isn't always appropriate with all music choices. Pianos sound lovely with the Primo 8, but if you crank them up too loud in order to enjoy the string bass more, then the piano might occasionally start to intrude too much.
I'm not saying that the midrange is aggressive per se, but rather it's that way in comparison to the more laid back Westone 4, and in relation to their bass output. If I try to remove anything in the midrange via EQ then something sounds like it's missing. It's better to add 2-3dB of bass than mess with the wonderful midrange, but 99% of the time I use no EQ at all to listen to these (but an amp with 10 ohm output impedance helps).
In direct back to back comparisons, even when amped the W4R can still sound a little less energetic and less exciting after having just heard the Primo 8. The W4 mids can also sound a little less rich and engaging in direct comparison. However, switching between these IEM is a slow process as the combination of the ear guides and the rotating cable jack makes it hard to insert the Primo 8 quickly. Plus I have to take time to unplug the IEM from amp and plug in the next one, and then volume match them. This slower process makes the memory of the previous IEM's sound fade a little, and complicates making impressions with back to back comparison.
Regardless, midrange clarity and vocal presence is very good, like the singer is in the same room with you. The Primo 8 seemed to do a little better job with male or female vocals than most of my other top tier universal IEM. The realism does improve as source and amp improve, and the midrange of the iPhone 5s built-in amp is not as good as that of the DAC/amps that I used (or even my iPad Air). Nevertheless, they are still very enjoyable un-amped.
However, with some songs via iPhone 5s headphone output, such as Maroon 5 "Payphone", the upper-mid/lower-treble could get slightly fatiguing, due to the combination with a less than perfect built-in amp. Basically you can hear the colorations in the iPhone 5s amp easier with the Primo 8, as they are not what I would call a very forgiving IEM. They show you just how good the mastering or the source and amp really are.
For use with the iPhone 5s I tried subtracting about 1.5-2 dB at 6Khz and that helped with this song, although it's not needed when using DAC/amps such as the HiFi-M8. And it's not needed with all songs via the iPhone headphone out, just some recordings. And I don't feel like the Primo 8 actually has a bump in the frequency response at 6Khz, because they don't seem to exacerbate sibilance. For example, with Diana Krall "Temptation" the song can sound sibilant with many headphones but not at all with the Primo 8. So this was clearly more of an issue with the iPhone's amp than with the IEM.
TREBLE & EVERYTHING ELSE:
The Primo 8's treble is smooth and extended. How extended I can't say because my 51 year old ears roll off a lot by 12Khz, and I have to turn the volume up a little with any headphone in order to hear the 16Khz tones. But 16Khz tones are as audible with the Primo 8 as they are with my other phones.
The W4R being a little more laid back or "mellow" sounding can make them feel a little more refined in the treble at times, but the Primo 8 don't seem to have any high-end grain. And they are transparent enough that they just seem to disappear at times.
Cymbals have great timbre and tone, with proper decay, and snare drums sound real rather than sounding plasticky or like an electronic drum machine. The treble detail and extension offers a great sense of air, ambience, and space with recordings; so even with the Primo 8's slightly forward mids the venue doesn't ever become small. Only your placement in relation to the performers is affected.
The W4R's less forward mids can make them seem a little more spacious than the Primo 8, but also slightly less intimate. Again, I'm not saying that Primo 8 have a small soundstage, but it's not exactly what I would call "holographic 3D surround sound" although the stage imaging is very precise with a wide stage.
I would say that the Primo 8 seem to put me on the 1st or 2nd row of the venue or auditorium, instead of 5-8 rows back like the W4 (but not on stage either). The W3 might put you even further back, while occasionally the Westone UM3X would put me on stage. The ambience from a live venue such as with Eric Clapton "Unplugged (live)" is actually quite good, lending to the realism of a large venue with a more intimate seating placement.
SUMMARY AND OTHER THOUGHTS:
People are always asking me, which universal IEM is best, or which one should I buy if I can only buy one. And my answer is usually there isn't one best headphone, but there's usually 2 or 3 complimentary ones that cover all the bases. As good as the HD800 or SR-009 are, they are complimented by also having an HE-500 or LCD-2 to rotate with them, maybe adding in a nice punchy closed headphone that isolates a little.
The same goes with IEM or earphones. And just like how different earpads or changes in fit on a headphone can change the sound from one person to another, so can the different IEM tips or changes in depth and position in the ear canal affect the sound.
So, what I hear with these IEM may not be what you hear. If I tell you what IEM to buy as your "one and only precious", there is a good chance that you will be happy and offer praise, and also a good chance that you will be disappointed. If I offer praise for an IEM and it doesn't sound that way to you, then you either have to change your tips, the positioning in the ear canal, your source/amp, or chalk it up to a difference in personal preference.
I want to make it clear that the Primo 8 is a very transparent sounding IEM, without disturbing colorations or veil over the details. Vocalists and instruments are "lifelike" and natural sounding. Often I listened to an entire album when I should have been doing individual song comparisons, forgetting that I was supposed to be evaluating their performance. They don't make it easy to get much work done while listening to them.
I really want to avoid calling them analytical, because some people would interpret that to mean they are cold or clinical, or without heart and soul. That couldn't be farther from the truth. But they certainly can be used to pick out details that may be skimmed over by other IEM. I don't think anyone would argue against these statements, but personal preference can influence how much one enjoys these traits.
In my case I think the Primo 8 are a great compliment to my Westone 3 or W4R (now the W30 and W40), or any other top-tier IEM with stronger bass. If I want to rock out to classic rock or electronic music I can go for the extra bass of the W4R, or even more bass with the W3. If I want to rotate to an IEM with a more laid back presentation then it may be the W4R over the W3, although the W3 still make a good low-volume/laid back IEM when used with Complys foam tips. But if I want to listen to a more balanced, detailed, transparent and realistic or lively presentation then I can pull out the Primo 8 as my first choice. This is where they really shine.
The Primo 8 are sometimes reminiscent of the Westone UM3X but as an upgrade, where they actually remind me more of my Westone ES3X custom IEM in terms of timbre and tone. I've always said that my ES3X were a great compliment to my Westone ES5 or JH16Pro custom IEM, which offer a little more warmth and impact. The ES3X have a similar life-like and detailed midrange with an energetic sound signature, although they also have a little more bass impact and larger soundstage - not surprising in a $800 multi-bore custom IEM vs a single bore universal fit IEM. In my ES3X review I also compared them to the HD800, for their similar sound signature.
In addition to audiophiles, I'm certain that there would be a large number of people out there that would want to use the Primo 8 as a neutral studio or on-stage monitor. Many other universal fit IEM don't have enough midrange to serve in that role, or they may offer a bit too much midrange to be enjoyed as much by audiophiles.
MORE ABOUT THE COMPARISONS:
As I mentioned before, the W4 and RE-600 are well balanced sounding competing IEMs that sound similar to each other with a good quality amp, but the RE-600 can only keep up with the Westone 4 if the RE-600 are amplified and not driven right out of the iPhone headphone jack. Using a HiFiMan HM-901 digital audio player, the W4 and RE-600 sound almost the same but with a slightly smoother treble in the RE-600 while there is an extremely slight grain to the W4, heard only in direct comparison.
The problem is that without amplification the RE-600 have less bass impact than the Primo 8, and the un-amped RE-600 sound fairly bland or lackluster in comparison. So, if both are un-amplified via iPhone headphone jack I would pick the Nuforce Primo 8 every time, although I'd still prefer to use my iPad Air. When I do add that extra little bit of bass (1-2dB) with an equalizer app, I can listen to the Primo 8 un-amped for hours and hours on end, with any genre or artist - it brings them closer to that "one and only precious" that I mentioned before.
Up until now the Westone 4 with their newer "Star" silicone tips were my preferred universal IEM because they have good sonic balance, strong bass, natural vocals, and a large soundstage while using the standard tips that they ship with. The W4 (W40) are one of the more accurate universal IEM on the market, but they can feel slightly veiled sounding vs a few other top tier IEM, making the W4 a little less exciting sounding than the W3 (W30). I was very happy with how well the W4 perform unamp'd with the newly designed stock tips, so it was a real eye opener to compare them to the Primo 8 and hear even more clarity and detail.
Nevertheless, I will not attempt to clearly rank the various universal fit IEM I've tried as being #1, #2, and #3 etc. That's very difficult because some IEM do certain things better than others, but then they lack in another area where their competition succeeds. I could go on and on about each and every universal fit IEM that I've owned or tried, about what I would change about them if I wanted to (I actually have that written up, but this isn't the time nor the place for that). So which IEM is #1 depends on what's more important to you.
FINAL CHARACTERIZATIONS AND RECOMMENDATIONS:
The Primo 8 have the overall detail of the W3, without the recessed midrange of the W3 or the slightly veiled midrange of the W4. The Primo 8 have a more crisp, detailed, engaging and lifelike midrange than the W4, but the mids can occasionally be a little too present and forward like with the UM3X (although the Primo 8 mids are more transparent). Basically, the W4 midrange levels are more balanced in relation to their bass and treble than the Primo 8 (more) or W3 (less).
When comparing the bass response and impact, the W3 might overdo things a little, while the W4 bass is just right, and the Primo 8 seem slightly weaker in their bass impact in comparison. On it's own, the Primo 8 is not a bass-lite IEM and the bass is fast, tight and extended. But the Primo 8 sometimes leave me wishing that I could crank up the volume for a little more bass impact without the mids becoming too loud at the same time.
The treble of the Primo 8 seems just right, while the treble of the W4 can be a little muted or muffled at times, and the treble of the W3 can be slightly hot or boosted when used with the wrong tips (grey silicone or triple flange). With Primo 8 the cymbals sound crisp and metallic, with better air and extension than the W4 and smoother than the W3. String instruments have the proper amount of texture in the bow on string, and voices do not become sibilant.
So, what does a person do when they like three or four different top tier IEM, but one is most balanced and detailed in the treble and midrange, another is most balanced sounding in the midrange and bass with slightly less detail, while the third has the best bass of them all but requires expensive custom fitted tips for the mids and treble to keep up, and the fourth requires an amplifier to keep up?
Well, I would tell a "bass-head" that W3 is #1 but they should only be used with Complys tips or UM56, since they can become slightly recessed, fatiguing or bright sounding with silicone tips. I would tell someone that W4 is #1 if they want a neutral monitor with strong bass at the expense of a slight decrease in midrange and treble clarity and openness. I would tell someone that Primo 8 is #1 if they want a warm and more energetic sounding neutral monitor, with the clearest vocals and instruments and a very open and transparent sound, as long as they don't need a higher-than-average or "boosted" level of bass impact.
In my case, I've pretty much moved on to high-end custom fit IEM for any serious listening at home, and I use my universal fit IEM for traveling to places where my IEM might get lost, damaged, or stolen. I don't want to be roughing it when I'm mobile, and with the Primo 8 I don't have to.