So after having the U12 for a month and a half now (or thereabouts), I finally feel good about heaping some compliments on this IEM. It's been a struggle for me to get the best out of it, and initially I was so unimpressed and disappointed in the sound that I was going to return it. Big mistake that would've been! First of all, I found fit quite tricky to get an ideal seal. It took experimenting at length with all the tips I have, and realizing that since the nozzle is so freakishly long that pushing the tips down as far as they'd go was the thing to do (and I mean so that the front edges of the nozzles are totally flush with the front surface of the tips). The tips I landed on are UE900 replacement single flange silicone. They have a wide open bore and allow the details and soundstage to really open up.
Sound-wise at first I felt there was a pretty hefty veil over the mids and highs, and a sense of smeared details (which I thought couldn't be right at all with all the positive things I'd read about the sound-- unanimously). But this seemed totally off to me, and like something had to be amiss in the mix of equipment. This was happening using the U12 with the Questyle QP1R, which again was bizarre because every other phone I'd used on it gave amazing sound. So I thought I'd just plug it into the basic iPhone 6+. Surprisingly that was much, much better and the details sharpened up and things got more open sounding. The only thing I can think was going on was an impedance mismatch possibly. Whatever it was, that pairing just didn't play well. I also had my ZX2 sitting around (had been collecting dust with the QP1R getting all the use of late), and great result out of that as well. Recently I picked up a SPC cable by the member here Wfanning, and wow does that give the U12 another nice jolt with detailing and space. So with the UE900 tips, and the Wfanning SPC cable, I'm definitely a happy camper.....
The thing that amazes me most of all currently about the U12 is the midrange. It is just so full, weighted, and properly placed... with far and away the best and most right tonality to vocals that I've ever heard from just about any phone. That midrange... jeez... glorious. I mean it kicks the snot out of the mids from the FL8Cs with the gold nozzle in place (and that was the best vocals/midrange by a good margin prior to the U12). The other thing I guess is how mellow yet detailed things are. There's never any glare, or sense it's trying 'so hard' to be ultra detailed. It's 100% easy to listen to for extended sessions.