The only thing Animagus left out of his review that I think is of significant importance is that I discovered that the KXXS absolutely requires 48-72 hours of break-in before they start to sound their best.
Up until owning the KXXS, I had never really noticed much of an improvement or even a difference when breaking in some of my other IEMs, so I didn't put too much stock in the belief that break-in can make a huge improvement in the SQ of a brand new IEM. Well I do now! I think the reason why may have to do with the DLC (diamond-like carbon) coating that is used on the KXXS's drivers. It makes them extremely rigid and that rigidity seems to require a significant amount of continuous break-in to get the drivers behaving the way the designers ultimately intend for them to behave.
When I first heard the KXXS at the SoCal CanJam, I thought they sounded great. So when my pair arrived I eagerly listened to them, only to be pretty disappointed by what I heard. While the FR was just as I remembered it, there was now this harshness to the sound that I did not remember hearing at all with the samples I tried at CanJam. The harshness was most apparent in the vocal range and it greatly exaggerated any small distortions in the recording in this range to the point that there was no way I was going to keep them if this was how they sounded.
Fortunately, I decided to try breaking them in by playing a few well-recorded vocal-centric tracks on repeat at a volume just below the loudest I normally would listen at. After the first 24 hours, I didn't notice any improvement and I thought that I would have to return them. But I let them break in for another 24 hours just to see if any change would occur, and thankfully it did! The unpleasant harshness in the vocal range was now almost completely gone! I listened to a bunch of my reference tracks and now they all sounded great, just like I remembered them sounding at CanJam. So I gave them another 24 hours of break-in and the sound improved a little more, but I didn't detect any significant improvements after the 72 hour mark.
So if you are considering getting the KXXS, just make sure to hold off on judging their SQ until they have had at least 48 hours of continuous break-in with music that has good dynamic range.
P.S. - Never use white noise or pink noise to break in headphones (or speakers). The nature of white and pink noise is pure RMS with no modulation. If played continuously at high levels, this type of sound can quickly cause a driver's voicecoil to overheat and warp, causing permanent damage. Just a little FYI.