INTRODUCTION
Our old friends at Knowledge Zenith (KZ) made waves in late 2018 with the release of the ZSN. The ZSN was a major upgrade from Knowledge Zenith's (KZ) very first hybrid IEM, the ZST.

But to understand where we are now (the ZSN Pro), we need to look at where we came from (the ZST and ZSN non-Pro).

The ZST was a single hybrid IEM, with 1xDynamic, 1xBalanced Armature per side. It had a medium-sized full acrylic resin shell, with a removable cable. It was a leap ahead of previous Knowledge Zenith (KZ) earphones, but it had serious flaws. The overall tuning was sloppy because there was no crossover. The treble was too sharp and sibilant for many people. So sharp, in fact, that I modified my BA drivers with a piece of acoustic foam (creating the 'ZST Foam Mod' on Head-Fi).

Fast forward to 2018, when Knowledge Zenith (KZ) announced a major upgrade to the original ZST (called the ZSN). The ZSN was based on the ZST shell, and maintained a single hybrid design - 1xDynamic, 1xBalanced Armature per side. However that's pretty much where the similarities ended. Everything was redesigned and upgraded from the original ZST:

• Revised shell, using better quality resin material
• Aluminum IEM nozzle and rear faceplate
• Revised 2-pin jack (now known as the Para C plug)
• Upgraded cable (braided, thicker, no more memory wire)
• Upgraded dynamic drivers (titanium plated)
• The addition of a passive crossover circuit

The ZSN was a major hit, and Knowledge Zenith (KZ) sold loads of them. In my opinion it was one of the best values in ChiFi for 2018. I still enjoy my ZSN to this day, and even handmade carbon fiber back covers for it.

However, nothing is perfect, and ChiFi is always evolving. The biggest complaint some people had was the upper midrange could be harsh/sibilant at times, most noticeable with female vocals. Despite this minor flaw, I didn't think the ZSN was so bad that it needed emergency attention (quite the contrary in fact). This is why it was a surprise when barely ~6 months later Knowledge Zenith (KZ) released the ZSN Pro.

So what's different about the Pro version, and is it better than the original ZSN? Let's dig deeper and find out.

SPECIFICATIONS

• Drivers: 1xDynamic (10mm), 1xBalanced Armatures (1×30095) per side
• Crossover: Yes
• Impedance: 24ohms
• Weight: 29g
• Sensitivity: 112dB
• Frequency: 7-40,000Hz
• Cable: 0.75mm 2-pin, removable, 3.5mm 90-degree plug, mic and no-mic versions available
• IEM colors: Black, cyan, purple
• Price: Roughly $14 (at the time of this review)
• Purchase link: ZSN Pro at Miss Audio

PACKAGING
The ZSN Pro comes in a small cardboard white box that is standard if you are familiar with Knowledge Zenith (KZ) earphones bought in the last few years. It’s nothing special, but it’s perfectly adequate and serves its purpose. You buy IEMs to listen to, not because they have a great box, right? The inner box has a clear plastic cover that shows off the ZSN Pro shells. And I know it sounds trivial, but I appreciate the small lip cut into the cover for easy removal. The IEM shells are mounted in an EVA foam layer, providing protection during shipping. The accessories (cable, spare eartips, and paperwork) are located underneath.



ACCESSORIES
The included eartips are all silicone type – 1 (pre-installed) pair of smooth silicone tips (black color, single flange, medium bore, M size), and 3 pairs of ‘Starline’ tips (black color, single flange, medium bore, SML sizes). Starline tips are one of my favorite tips, as the black ones are made of soft and flexible silicone, and they fit well enough for all day comfort.

The cable is a removable design, with 4-wires in a braided configuration. The ZSN Pro is available with and without an in-line microphone (mine being the no-microphone version). The microphone version has a single control button, providing music and phone call control (but no volume control). The plugs that attach to the IEMs are clear plastic, with L and R indicators (very difficult to read). The termination uses a 2-pin (0.75mm) design, with a new raised design not seen on any previous Knowledge Zenith (KZ) IEMs. This new 2-pin plug type is officially referred to as 'Para C' or 'Paragraph C'.

In case you aren't familiar with Knowledge Zenith's (KZ) 2-pin nomenclature:

• 'Para' A is for ZS3, ZS3E, ZS4, ZS5, ZS6, ZS7 (& others)
• 'Para' B is for ZSR, ZST, ZS10, ED12, ES4, ES3, CCA C16 (& others)
• 'Para C' is for ZSN, ZSN Pro, CCA C10, ZS10 Pro (& others)

The cable itself is a good balance of being not too heavy/thick, but not too light/thin either. It’s brown in color, and has good strain reliefs at the Y-split and 3.5mm end. The 3.5mm end is a 90-degree L style plug. The cable also has pre-formed ear guides, which are secure yet flexible. This is a welcome departure from the annoying bendy memory wire Knowledge Zenith (KZ) has been using for years. Thank you Knowledge Zenith (KZ)!



It is one of the nicer ‘upgrade’ cables I’ve seen on a stock Knowledge Zenith (KZ)/CCA cable, and the pre-formed ear guides alone make the cable worth it. My only gripes with the cable are the lack of a chin slider and how low the Y-split is located, both of which can make the cable prone to tangling. I’d love to see a factory-applied chin slider at some point in the future, but for now you can add one of your own. A small rubber o-ring works well as a makeshift chin slider, which helps prevent tangles considerably.

BUILD
The Knowledge Zenith (KZ) ZSN Pro has an acrylic (polymer resin) shell that comes in 3 transparent colors – black, purple, and cyan. You can see the internals of the ZSN Pro through the shell, although the BA driver is located in the nozzle and can't be readily seen. The back cover is made of a zinc-aluminum alloy, and has a matte finish (silver or black depending on the shell color selected). The cover has 3 raised zig zag lines, 1 small bass vent, and 1 mounting screw.



The ZSN Pro a substantial and premium feel. The resin shell is very smooth, and there are no sharp edges. It’s very comfortable to wear, even for extended periods. The overall fit and finish is excellent, with no mismatched seams, gaps, or excess glue.

The shells have no channel indicators, and the only thing written on the side is “ZSN-PRO Classic Upgrade". While it would have been nice to have channel identifiers on the IEM shells, here's a handy guide showing the correct orientation of the shells and cable:

1. Hold up the IEM shell so that the 'pointy' part is facing down.
2. Now look at what direction the nozzle/eartip is pointing.
3. If the nozzle/eartip is pointing to the LEFT, then that's the LEFT IEM shell.
4. If the nozzle/eartip is pointing to the RIGHT, then that's the RIGHT IEM shell.

The ZSN Pro's nozzle is 6mm wide, and is what I would consider an average/medium length. This allows either a shallow or deep insertion, depending on the eartip used. There is a retaining lip on the nozzle, allowing the eartip to stay securely on the IEM nozzle with little risk of falling off or staying behind in your ears while removing the earphone.



SOUND
The gear used for testing included: xduoo X3 (with Rockbox firmware), iPhone SE (with both headphone jack and Apple lightning adapter), and FiiO E12A Mont Blanc headphone amplifiers.

The ZSN Pro was easy to drive with any source, and I did not really notice any improvement when using an amplifier. This makes it ideal to use on-the-go with nothing other than your phone.

Bass
The ZSN Pro uses a new double-magnet (tesla) titanium plated dynamic driver. KZ claims this improves the control as well as the dynamics of the driver. And I agree with this statement. The most obvious improvement with the ZSN Pro over the original ZSN is the sub bass. It both extends deeper and is more powerful than the original ZSN. If you listen to bass-oriented genres you'll definitely appreciate the change. The low end has a satisfying rumble, which you'll appreciate if you're a basshead. There is some bleed into the midrange however.

Midrange
Overall, the ZSN Pro’s midrange is pleasing and warm. The midrange is recessed, and as a result male vocals aren't as strong as they could be. However, this is no surprise given the ZSN Pro’s v-shaped sound signature. The upper midrange (~3kHz) have been somewhat dialed down and smoothed out compared to the non-Pro ZSN, which I feel was KZ's attempt to address the complaints of midrange harshness some people had with original ZSN. While I definitely notice the difference this tuning change made, I never really had an issue with the midrange tuning of the original ZSN to begin with.

Treble
Treble on the ZSN Pro is pretty bright overall, with some airiness. Knowledge Zenith (KZ) actually added some additional treble to the ZSN Pro (vs the non-Pro ZSN), which does a nice job to balance the increased low end of the new double-magnet dynamic driver. If you're extremely treble sensitive, you may have issue with the ZSN Pro. Otherwise, the increased treble adds extra clarity compared to the non-Pro ZSN. There is some minor sibilance, but I found it track dependent as opposed to being inherent in the drivers or tuning.

Isolation
The isolation is above average, which I attribute to the combination of the ZSN Pro's heavier zinc alloy rear cover and single vent hole (vs 3 vents for the non-Pro ZSN).

Soundstage
The soundstage is average in depth and above average in width, both of which are improvements over the original ZSN. Imaging and instrument separation are both good, especially given the ZSN Pro's price.

CONCLUSION
The Knowledge Zenith (KZ) ZSN Pro definitely makes some improvements on the original ZSN. However, the improvements are not massive, but rather subtle and incremental. Whether or not you should get it depends on if you already own the original ZSN or not. If you don't own either, I'd recommend going with the ZSN Pro. It builds upon its predecessor, with an upgraded driver, improved dynamics, more treble clarity, wider soundstage, and better isolation. However, if you already own the original ZSN you may want to consider the ZSN Pro only if you're one of the people that felt the upper midrange was too harsh, or if you wished the ZSN had a more elevated low end.

DISCLAIMER
I would like to thank our friends at Miss Audio on Aliexpress for providing the Knowledge Zenith (KZ) ZSN Pro for my independent review.

This review was originally posted at https://audioreviews.org.

CCA C10

Introduction:

If you’ve never heard of CCA until recently, you’re not alone. CCA stands for Clear Concept Audio. They came out of nowhere in 2018, and have been steadily releasing a number of budget earphones ever since. CCA models are built well, have appealing tuning, and are delivered at a budget friendly price.

Well, as it turns out, CCA is a sister company of the budget earphone kings: Knowledge Zenith (KZ). This has both pros and cons. On the one hand, Knowledge Zenith (KZ) has a long history of producing solid performing earphones. They’ve perfected their manufacturing practices and provided killer bang for the buck value. In addition, manufacturing facilities, components, and engineering experience can be shared between the 2 companies. However, at the same time this creates additional saturation in an already saturated ChiFi market. With this saturation is the possibility of ‘side-grades’, caused by the overlap of seemingly similar models with similar tuning between the 2 companies. Regardless, I’m excited for the future, and only time will tell what the long-term outcome is.

With that said, let’s take a closer look at the C10 and see what it has in store for us.

Specifications:
Drivers: 1xDynamic (10mm), 4xBalanced Armatures (2x50060, 2x30095) per side
Crossover: Yes
Impedance: 32ohms
Weight: 29g
Sensitivity: 108dB
Frequency: 7-40,000Hz
Cable: 0.75mm 2-pin, removable, 3.5mm 90-degree plug, mic and no-mic versions available
IEM colors: Black, cyan, purple

Packaging:
The C10 comes in a small cardboard white box that is standard if you are familiar with Knowledge Zenith (KZ) earphones bought in the last few years. It’s nothing special, but it’s perfectly adequate and serves its purpose. You buy IEMs to listen to, not because they have a great box, right? The inner box has a clear plastic cover that shows off the C10 shells. And I know it sounds trivial, but I appreciate the small lip cut into the cover for easy removal. The IEM shells are mounted in an EVA foam layer, providing protection during shipping. The accessories (cable, spare eartips, and paperwork) are located underneath.



Accessories:
The included eartips are all silicone type - 1 (pre-installed) pair of smooth silicone tips (black color, single flange, medium bore, M size), and 3 pairs of ‘Starline’ tips (black color, single flange, medium bore, SML sizes). Starline tips are one of my favorite tips, as the black ones are made of soft and flexible silicone, and they fit well enough for all day comfort.

The cable is a removable design, with 4-wires in a braided configuration. The C10 is available with and without an in-line microphone (mine being the no-microphone version). The microphone version has a single control button, providing music and phone call control (but no volume control). The plugs that attach to the IEMs are knurled aluminum with color coded indicators (blue=left side, red=right side). The termination uses a 2-pin (0.75mm) design, with the same ‘keyed’ design shared with other Knowledge Zenith (KZ) IEMs. This ‘key’ has a flat end and a curved end to help indicate what the proper orientation of the plug is. The cable itself is a good balance of being not too heavy/thick, but not too light/thin either. It’s iron grey in color, and has good strain reliefs at the Y-split and 3.5mm end. The 3.5mm end is a 90-degree L style plug. The cable also has pre-formed ear guides, which are secure yet flexible. This is a welcome departure from the annoying bendy memory wire Knowledge Zenith (KZ) has been using for years.

It is definitely one of the nicer ‘upgrade’ cables I’ve seen on a stock Knowledge Zenith (KZ)/CCA cable, and the pre-formed ear guides alone make the cable worth it. My only gripes with the cable are the lack of a chin slider and how low the Y-split is located, both of which make the cable quite prone to tangling. I’d love to see a factory-applied chin slider at some point in the future, but for now you can add one of your own. A small rubber o-ring works well as a makeshift chin slider, and this helps prevent tangles considerably.



Build:
The CCA C10 has an acrylic (polymer resin) shell that comes in 3 transparent colors - black, purple, and cyan. You can see the internals of the C10 through the shell. The back cover is made of a zinc-aluminum alloy, with a matte silver finish and the CCA logo with “BAx8 DDx2” printed on it.



Zinc alloy is heavier than aluminum, but the zinc alloy back cover gives the CCA C10 a premium heft, without being overly heavy to wear. The C10 body shares the same shell as the Knowledge Zenith (KZ) ZST, ZSN, and ZSN Pro. The only differences with the shell are:

• The ZST has an acrylic shell, acrylic nozzle, and acrylic back cover. It is the lightest of the 4 IEMs, partly due to the 100% acrylic construction, and partly due to only having 1xDD and 1xBA drivers.
• The ZSN has an acrylic shell, aluminum nozzle, and aluminum back cover. It is heavier than the ZST, mainly due to the aluminum nozzle and back cover.
• The ZSN Pro has an acrylic shell, aluminum nozzle, and zinc alloy back cover. It is heavier than the ZSN, mainly due to the zinc alloy back cover.
• The C10 has an acrylic shell, acrylic nozzle, and zinc alloy back cover. It is heavier than the ZSN Pro, mainly due to the 3 extra BA drivers compared to the other ZSx models.

The shell is very smooth, and there are no sharp edges. It’s very comfortable to wear, even for extended periods. The overall fit and finish is excellent, with no mismatched seams or excess glue gooped everywhere. Each shell has its own channel indicators, written on the side as “C10-Right” and “C10-Left”, along with “10 Hybrid technology”.



CCA really wants to make sure you didn’t forget that the C10 has 10 drivers, as they wrote it not once but twice. As much as I can joke about it though, just 4 or 5 years ago this would have only been a pipe dream when speaking about budget earphones. It really shows how far things have progressed, and we are very fortunate to have access to this level of earphones nowadays.

The nozzle is 6mm wide, and is what I would consider an average/medium length. This allows either a shallow or deep insertion, depending on the eartip used. Speaking of eartips, there is no eartip lip on the nozzle. This is a peeve of mine, because the eartips can slide around on the nozzle (making the eartip crooked and affecting fit). Or even worse, the tip can pop off and stay behind in your ears when you remove the earphone. I don’t know why manufacturers don’t just put a lip on every single earphone without fail. There is absolutely no logical reason not to have a lip on every IEM nozzle.



Sound:
So how does the sub-$50 CCA C10 sound? In 1 word, impressive. But let’s see what that means exactly.

First I’ll mention the gear used for testing: xduoo X3 (with stock and Rockbox firmware), iPhone SE (with both headphone jack and Apple lightning adapter), iPod Classic 5G with Rockbox (with both headphone jack and line out dock adapter), FiiO E12A Mont Blanc and FiiO A5 headphone amplifiers.

Despite being 32ohms, the CCA C10 is easy to drive without using an amplifier. In fact, I really didn’t notice any improvement when amped. It sounded fine on all sources, including right from my iPod and iPhone SE.

As far as the sound profile, the C10 are definitely not as v-shaped as most Knowledge Zeniths (KZs).

Bass
The sub bass hits hard when called upon, and has deep extension. But at no time is it overwhelming. The dynamic driver’s bass is smooth and linear, but I wish it had the kind of instant speed possible from balanced armature bass drivers (required for genres like Heavy Metal and Industrial). Luckily there’s no mid-bass bleed, which is a common issue with a lot of budget ChiFi (not so with the C10). Overall, the bass has a warm presentation, which is meaty and satisfying.

Midrange
I was relieved when I first heard the C10, as I was expecting the deeply recessed midrange that’s all-too-familiar. Not so with the C10. We get a nice thick midrange with forward vocals. Brass instruments and acoustic guitars sound great, and are perfect for jazz, folk, and country genres. Just like the bass, we get nice warm mids without the tinniness that can often ruin otherwise good midrange.

Treble
Although the treble of IEMs like the ZS6 never bothered me, I get the feeling the folks at CCA/Knowledge Zenith (KZ) learned their lesson with regards to aggressive treble. There is plenty of sparkle and air, without treble being harsh or fatiguing. There’s also no ssssibilance, thanks to a dip between 6-7kHz. Upper treble does roll off, which is apparent in crash cymbals for example. Treble that’s good without being bad can be a fine line to walk, and the C10 pulls it off nicely.

Isolation
The isolation is excellent, with minimal sound leakage. There are vents on the C10, but they are both facing inwards against the ear. The isolation isn’t as extreme as the Knowledge Zenith (KZ) ZS3, ZS3E, or ZS4, but you have to really crank the volume on the CCA C10 for external users to hear your music leaking out.

Soundstage
The soundstage is average in both width and depth. It reminds me of a larger-than-average night club venue vs a cavernous cathedral. Sorry, no 3D holographic ‘in your head’ feeling many people look for. However, imaging is good. I would have liked to have slightly better instrument separation.

Conclusion:
If you’re sick of the typical deeply v-shaped budget ChiFi, and are looking for something a bit more balanced with vocals that shine, the C10 is a great option. With a warm and smooth presentation, it’s relaxing and easy to listen to (especially with low-fi sources like music streaming apps on a phone). Amping is not necessary to get the best out of the C10, which makes it an easy choice for on-the-go mobility. Midrange is forward without being dominating. The low end digs deep, with an authoritative rumble. And treble has sparkle without being harsh or too aggressive. It’s excellent for genres such as EDM, pop, jazz, and country.

BQEYZ KB1



Specification:

• Price: $50
• Design: In-ear, worn up (behind ear)
• Housing: Aluminum
• Colors: Black, Grey
• Nozzle size: 5.5mm
• Drivers: 10mm single dynamic + 6mm single dynamic + 1 Balanced Armature
• Frequency response: 7Hz–40kHz
• Impedance: 15ohm
• Sensitivity: 105dB
• Cable: Available with wired cable OR wireless Bluetooth cable
• Removable cable: Yes
• Connector Type: 2-pin, 0.78mm (compatible with 0.75mm)
• Controls: 1 button control w/mic (wired cable) OR 3-button controls w/mic (Bluetooth cable)
• Weight: 25g
• Accessories: 3 pairs single-flange silicone ear tips (S/M/L), micro USB charge cable, user guide
• Warranty: 12 months

Where To Purchase:
https://www.amazon.com/dp/B07HLTLDBJ

Disclaimer:
Thanks to Elle at BQEYZ, who provided the new KB1 model in exchange for an honest review. And when I say honest, I mean honest. If I don’t like something, I’m not going to candy coat it (I just won’t review it). This review is my own, and does not represent BQEYZ or any other entity.

Company:
BQEYZ is a relatively new brand to many ChiFi enthusiasts. However, the reality is that they have been around since 2003 as an ODM for other companies. They only recently started marketing their own BQEYZ brand.

Introduction:
The BQEYZ KB1 is a triple driver hybrid IEM. BQEYZ offers it in 2 different configurations - wired and wireless. I am reviewing the Bluetooth cable version.

Packaging/Presentation:
The box is a simple black cardboard box with BQEYZ branding on the front and product specs on the back. Inside is a foam tray, which holds the shells and the spare silicone ear tips. Under the tray is the Bluetooth cable, micro USB charging cable, and user manual. It is a compact and sparse presentation, but perfectly adequate (and typical for ChiFi offerings at this price point).

Internal Design:



The KB1 is a triple driver hybrid IEM, containing (2) separate single dynamic drivers, as well as (1) Balanced Armature. The 10mm dynamic driver handles the sub-bass and lower bass frequencies, the 6mm dynamic driver handles the midrange, and the BA driver handles the treble frequencies. The 2 dynamic drivers are arranged in a side-by-side configuration, with a tuned sound pipe running between them. The balanced armature is mounted in the nozzle (directly under the nozzle screen), and does not use any tubes or dampening resistors. BQEYZ uses unique balanced armatures in their IEMs, which I have not personally seen on any other ChiFi IEM. The output nozzle is copper, which helps control high frequency ringing. It is unknown what diaphragm material is used on the dynamics, nor if the IEM uses any passive crossover circuitry (although it is likely based on the coherency of the tuning).

Build:
The KB1 shells are made from CNC machined aluminum, with a sand-blasted surface. The color is electro-anodized (black or grey), so it will not chip off like painted IEMs. Anodized finishes, while tougher than paint, can still scratch and wear off if abused. I recommend using a carry case (not provided) to store the IEM when not in use, to minimize wear to the finish and maintain their long-term appearance. Each shell has L and R engraved on the front, so it is easy to tell which side is which. The nozzles are angled, 5.5mm wide, and have no lip. I had no issues with ear tips falling off, however I always prefer lips on my IEM nozzles whenever possible. The shell halves are joined together tightly and evenly, with a clean seam and no noticeable gaps. The back is not removable (ie no screws).

Fit/Comfort:
The KB1 has a kidney bean shape, with flat faces and sides. All edges have a 45 degree chamfer, eliminating any sharp edges. The fit is comfortable and sits flush with my ears. I experienced no hot spots while wearing the KB1. However, while the chamfered edges improve comfort, I prefer the totally smooth and rounded edges found on the BQEYZ K2 and KC2.

Connectivity:
The included Bluetooth cable is a braided cable (black color), using silver plated copper conductors. Bluetooth is version 4.2, and it also offers both aptX as well as aptX-LL (low latency). Many Bluetooth cables exclude aptX-LL, which is unfortunate because it allows an excellent low-latency experience when watching movies and videos.

The cable is soft and resists tangles, with pre-formed ear guides at each 2-pin connector. The plastic Bluetooth control box (located on the right side), and has a microphone and 3 control buttons (power/volume/music control). The exact functionality of the buttons will depend on your source and/or apps.

I had zero issues with disconnects or dropouts while using the KB1. The range is typical for these inline Bluetooth cable designs, which was about 10 feet of rock solid performance. This range is adequate when using a laptop/tablet, when your phone is in your pocket or by close by during a workout, sitting next to you on a bedside or kitchen counter/table, etc.

This style of cable is not realistically designed to push the extreme boundaries of Bluetooth's range (10M=30ft). The manufacturers maximize battery life by keeping output power as low as possible. My environment is a typical residential house in a typical suburban neighborhood (w/wifi networking and numerous other wireless devices). If you live in a densely-packed urban environment with significant sources of radio interference, YMMV. As long as you understand Bluetooth's capabilities and have realistic expectations, you will be fine.

Pairing was quick and easy, and all prompts are in English (ie "power on", "pairing", etc). There is a small LED that indicates power, charging, pairing, etc. The battery lasted between 3-4 hours between charges, but your results may vary depending on factors such as max volume and codec used.

Additional note regarding the cable: The included cable can be swapped for any 0.75mm or 0.78mm 2-pin cable (including other BQEYZ cables, KZ/TRN cables, etc). With an impedance of 15ohm & 105dB sensitivity, the KB1 is easy to drive using a cell phone or laptop when used with a 3.5mm cable.

Sub-Bass & Bass:
The KB1 has a v-shape tuning, with a focus on bass but without being exaggerated. Sub-bass has deep extension, and the bass is tight and punchy. There is some mid-bass bleed, however it can be eliminated with EQ.

Midrange:
Midrange is crisp and warm, and has a refreshingly natural sound with no coloration. The midrange is recessed, but this is expected for a v-shape tuning. Despite being recessed, vocals don't sound overly distant like you often get with some v-shaped gear. The mid-bass bleed is the only thing holding the KB1 back from having superb mids, and I hope BQEYZ works on eliminating that from future IEMs. This is not the only BQEYZ to suffer from mid-bass bleed.

Treble:
As with the BQEYZ K2, I really like the treble on the KB1. It is detailed, airy, and well balanced with the low end. Treble is bright without being harsh. I experienced no tiring or fatigue, even after long listening sessions. Clearly BQEYZ made an excellent choice with the unique copper nozzle Balanced Armature driver.

Imaging & Soundstage:
Imaging is good, allowing instruments to be clearly identified. The soundstage also surprised me - the large size makes this above average. A lot of IEMs have a compressed soundstage; not so with the KB1.

Leakage/Isolation:
The sound leakage and isolation is better than average. This is attributable to the aluminum shell, as well as to the vents (which are all located on the ear-facing side, vs facing outwards). Plastic IEMs and IEMs with side or rear vents usually have poorer sound leakage and isolation.

Negatives/Gripes:
If I could make any changes, it would be eliminating the mid-bass bleed, adding a lip to the nozzle, and rounding the shell edges (instead of chamfering). These are minor gripes though, and do not detract from enjoying the KB1.

Summary:
The BQEYZ KB1 offers an impressive value-to-performance ratio. It has a quality all-metal construction, comfortable fit, detachable cable, and solid Bluetooth performance (<10ft).

Those that enjoy bass-intense musical genres will be rewarded. The bass hits deep and hard, with authoritative punch. The warm mids deliver natural vocals and pleasingly crisp guitars. The treble is bright, resolving, and balanced, without overpowering the signature (like the treble-forward KZ ZS6). While the isolation/leakage isn't as extreme as the KZ ZS3 or ZS4, you won't have any issues when listening around others (unless you crank the volume to ear-bleedingly high levels).

Coming from the BQEYZ K2 (which I already owned), I was already thoroughly impressed with BQEYZ. They have (so far) consistently demonstrated a capability to make quality products with competent tuning.

If you are looking for a solid option in the ~$40-$50 budget range, I recommend these.