Wow, lots of U12t talk. I guess I have to drop my 2 cents as the resident U12t shill. At this point, I've put hundreds of hours onto my U12t, so I think I have a pretty good feel for where I stand with it.
eye-candy so you don't have to stare at my wall of text
The U12t has the best BA bass I've heard, but at the end of the day it's a compromise. By default, it's never going to quite match the best DD woofers. The U12t's midrange is ridiculously safe, but that's because it chooses to dip the upper-midrange which some might find sounds overly dampened. That's a compromise. It has terrific treble extension and air, but that's because of the tia tweeter's crazy treble peak. It's totally unnatural relative to what you'd hear in-real-life. Again - a compromise. Technicalities, I won't say it's not a very technical IEM, but the softer attack transients definitely take the edge off of perceived resolution. Furthermore, the cleaner, more natural pattern of decay the U12t exhibits, to eschew BA timbre, is a trade-off. To this end, I've found myself second-guessing its microdynamic ability lately, especially relative to the HD800S and Clear I have on-hand. The U12T has decent micro-detail for an IEM to be sure, but more intimate fluctuations themselves seem a tad smoothed over, particularly in terms of resonance.
There's a prevalent theme of middle-ground at hand, and I really do think that the U12t is the epitome of it in many respects. I've said this before, but the U12t's that kid in school who gets a 90% in every subject, but can't seem to get 100% in everything. If you want the best bass in an IEM then there's IER-Z1R, if you want the best midrange for male vocals then there's the VE8, if you want the best treble, again, there's IER-Z1R, and if you want the best technicalities go buy a KSE1500 or VX. As the type of person who always wants to own the best, that was one of the hardest things to come to terms with in this hobby. IEMs
themselves, from inception, are a fat compromise in that they'll not match a headphone for many aspects of sound production.
But enough ragging on my favorite IEM. Aside from the tuning, I would say a large part of the reason I keep coming back to the U12T is for the impressive dynamic range. The way it scales and pushes into macrodynamic swings is incredible for an IEM, and most everything else I’ve heard comes off as somewhat flat by comparison. This is what largely qualifies so-called "engagement factor" to my ears (again, for me only). To come off of something like the Focal Clear, a headphone known for its terrific macrodynamic punch, and not be immediately disappointed is impressive. And then there's the U12t's center image which I don't hear talked about often. The U12t's upper-midrange recession and post-10kHz treble contrast create the psychoacoustic illusion of the sound actually projecting in front of a listener in a speaker-like manner. It's uncanny, and intended by 64 Audio or not, distinctive to me of what the right synergy between tuning and intangibles can pull-off. Heck, I've yet to hear a
headphone that does center image like the U12t can. This is one IEM I don't think I'm ever going to be able to stop talking about (and yes, I've literally gotten complaints from people dreading their favorite IEM being compared to the U12t). Like so, for a lot of people it'll just be the IEM that gets the most done right and makes the least mistakes. And you know what? I'm cool with that because that's pretty darn commendable of itself too.
I think what they mean by stiff is in comparison to the traditional, braided cable. The OSLO's conductors have that single-pipe design that doesn't allow for the same bends as a multi-strand, braided cable does. I personally like it better, because it doesn't retain memory or kink as easily. But, I can see the flip side as well.
