This shouldn't be a huge surprise when I tell you that the KB1 are basically a K2 minus one of the balanced aramtures. In fact, the sound is so similar that I probably wouldn't be able to tell them apart in a blind test.
One thing I noticed, is that the KB1 is slightly (barely noticeable) more relaxed upper-highs. This made them extremely listenable for long sessions with absolutely no fatigue. They are also slightly less bulky than the K2, which means they are relatively more comfortable.
Another improtant thing to mention is that this time, wide-bore ear tips are included with the KB1, and those fit the nozzles perfectly. That's a huge deal, since the ear tips I received with the K2 were narrow and barely fit the extra-wide nozzles.
Aside from that, there isn't much difference between these products, and it's really up to you to decide which to get. Just remember, with the KB1 I didn't have to replace the ear tips, which made the whole experience smooth and instant. I think that you won't be missing out on anything without that extra balanced armature. The BQEYZ are indeed a serious brand and each aspect of their products portrays top-notch quality.
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Impressively clean, balanced and addictive sound quality, especially for vocal-centric music.
Extremely loud and easy to drive.
Quite comfortable and may even fit smaller ears.
Unbeatable build quality. Hefty housings for a trustworthy sturdiness.
The cable is superb and the pre-shaped ear loops are very convenient.
Instances of pierce are still a possibility on susceptible tracks with sharp high-pitched sounds, however only those with highly sensitive hearing will be able to notice.
The nozzles are quite short, which might cause fitting issues for some users.
No additional accessories, like a shirt clip or a carrying case, considering the price tag.
A huge improvement over K2 in terms of size of the housings, and the included ear tips. The nozzles are a bit short, so I had to adjust them a bit to get a proper seal. Thanks to the medium size and the wide-bore tips, you will have no issues fitting these earphones, even if you have smaller than average ears.
Lows / Bass
Once the ear tips are changed to some wide-bore ones, the bass gets quite extended and very pleasing. In fact, the bass quality and extension seems to be almost identical to RevoNext QT2, but just almost, as QT2's bass has more body, it is warmer and more energetic.
Mids / Vocals
Extremely similar to RevoNext QT2 and I could barely tell them apart. They are not recessed at all, but are also not edgy. The perceptional distance of voices and instruments from yourself is phenomenal. You will feel surrounded with sounds and will also be able to tell the direction from which the sounds originate. Not only that, the mids also sound extremely natural without any coloration, at least as far as I can hear. Those mids are as universal as it gets!
Highs / Treble
Also very similar to RevoNext QT2 and hardly distinguishable. Very detailed, crisp and sharp, but not fatiguing. Obviously, certain tracks (specifically under the electronic music umbrella) will still cause some pierce and it seems inevitable when it comes to bright earphones, especially those with balanced armatures located in the nozzles. However, those are some of the most extended and accurate-sounding highs I've ever heard so far. Certainly, you will get more sparkle with the extremely-extended treble of ZS6, but K2's highs will not tire you. Compared to the similar TRN V20, K2's treble is more accurate and detailed, less rolled-off but instances of pierce are far less likely to occur - and that what makes them truly worth their extra price.