Just a quickie because I totally lack impulse control...
 
 Good sound quality for the money. Bass is slightly elevated. A tad soft but ok texture. The overall signature in passive is smooth and warm. A bit similar to stock UE900 but a bit warmer. Treble is generally soft but with some sparkle. Comfort is good and build quality is good although it looks a bit spaced out for my taste. The active mode adds some hiss and bumps the signature to more of a V-shape, It does remove some of lower frequencies which is most noticable on buses or trains or similar environments. Can make lower treble/upper mids sound a tad harsh on some recordings. 
 
Overall a great product and a steal at <$100. 

I'm going to write a new, proper, monster review of these later on. Just HAD to write something now. This is sort of in between my old review and my upcoming one.
 
Useful iem tips; Comply TS400/500, Auvio tips, Meelec Balanced tips. (All of these are better than the stock tips imo.)
 
The bass: Depending on tips, it hits hard, harder or very hard. My preferred tips are Meelec balanced dual flanges. They give the least mid bass impact, also the least bass bleed i.e., none to my ears. The sub bass is great and sounds realistic. Not overdone. The texture is great and bass guitars really shine if placed forward in the mix, however less so with the meelec tips. Using Comply TS400/500 (both fit actually) give a more closed in sound but it's still better than the included T400 (or 500, don't know). It brings a nice Comply-ish texture to the bass. More mid bass. More impact but more bleed. Auvio tips give the strongest bass response to my ears. They lack the texture of the TS500 but I'm fairly certain that Comply tips themselves imply a kind of texture/sound to the bass regardless of iem.
 
The mids: Here's where it differs the most. Again, my preference is Meelec balanced tips. With them the mids are quite leveled. The upper mids are just a tad smooth. Nothing that's getting to me really. I find the lower mids to be in line, not too warm. Male voices do not sound overly full or bloated. They sound really refined actually. They sound like I imagine a large membrane microphone sounds like. The detailing is really, really, really good. Until I got these, I craved the crisp upper mids of the CK100Pro that I borrowed and reviewed. The memory of them had really etched itself in my brain. Now I don't even consider buying them anymore. That's the Meelec balanced tips. The TS500 give a weightier sound. They sound fuller and more laid back. Less spacious and open. A bit more blurred but still top level details. The upper mids do get overshadowed by the lower mids, they don't have the same bite as they do with the meelec tips. Strings, brass and electric guitars sound a bit muted/dull/uninspiring/sleepy. Still better than UE900 and more presence than ASG-2. But it lacks that little bite that ASG-2 has despite it's overall darker tonality. Still, very solid choice of tips for a laid back listening. Auvio tips give the strongest bass response and the strongest mid bass which colours the mids. Some may like it. I find that the increased presence of the lower mids veil the upper mids. The treble is more present than the other tips and there can be some sibilance. Some prefer these tips, I like the more neutral meelec tips.
 
The highs: The highs are smooth, detailed, well extended. They aren't the airiest highs I've heard because they belong to the CK100Pro. But they are almost never sibilant. Great detail. With meelec balanced tips the balance between fundamentals and overtones in cymbals are very good. With TS500 there is a lack of fundamentals and the overtones are a bit muted. With auvio tips there is still a bit of fundamentals lacking and the overtones are stronger. My vote again falls on the meelec balanced but that's probably because I've figured out that I like to hang out just around neutral (but with stronger sub bass/lower bass).
 
Soundstage and imaging: Great soundstage and imaging. The claustrophobic feeling with T400(or 500, still not sure) that are preinstalled, is remedied easily with any of the other tips. Again, meelec balanced tips (<3). They give an openness to the soundstage and a better sense of air. The soundstage isn't as open as the IE8 (from memory) but the isolation, separation, detailing and everything else is just slaughtered by the FA-4E. I'd say that with a good source there is a nice 3D sound. Out of a smartphone it's mostly width [please insert name of iem that doesn't sound 2D out of a smartphone]. The separation is great and so is the placement of instruments. Switching to the ASG-2 does widen the stage a bit, could bring more height, I'm not sure. But it also tends to make the sound more Left-Center-Right. Instead of a nice panorama that fades into another part of the stage nicely. 
 
Summary: The FA-4E is quite similar to the UE900, only better. I can't think of one single thing that the UE900 does better. Well, it's more functional with two cables included. Has a nicer case (the one that looks like you could pop a wedding ring out of it. And it does have sort of a more mainstream visual appeal to it. But the technical prowess of the FA-4E is a few notches over the head of UE900. The bass is tighter, the mids are more detailed and have a better texture/timbre, highs are pretty similar except that the UE900 does have a tendency to show sibilance in a few songs that the FA-4E doesn't. Fischer Amps has created a wonder. Or a monster. That's for sure. My hat off to Fischer Amps for creating one of the best universal iems regardless of price (more of a conclusion made from various posts in the thread) and one of the strongest contenders in its price bracket.
 
Simon

The following is taken out of its context from my review here http://www.head-fi.org/t/687962/asg-2-ck100pro-and-ue900-reviews-the-dysfunctional-family I just thought that the CK100Pro deserved a review here. I don't own it but have auditioned it extensively over about 8 days. Though I just might buy it later...
 

First of all. I'm going to review these based on my listening experiences using a 100 Ohm adapter or from the Out2 of my BH. My pinholes are about 2/3 - 3/4 blocked. I'm using meelec balanced dual flanges and I have elongated the nozzle of my right housing with the tube from a comply TS500. For me that's the best sound I can get from them.
 
These feel like they don't need any formal introduction. As the follow up and replacement to one of the most renowned iems on the planet, the TF10, they have a lot to live up to. I've never heard the TF10. The UE900 were actually my first real iem-crush. From the moment I saw them I knew that I would buy them. With every review I read I knew that they were probably not the best price/performance and certainly not the best build quality for the price. Before I bought them I knew that I wanted more of a neutral sound with extended treble. http://rinchoi.blogspot.se/2012/10/ultimate-ears-part3-in-depth-analysis.html That link is probably what tipped me and made me pull the trigger. A choice that I don't regret.
 
Build, package, comfort: The included package is good. The best part of it is the airline attenuator. If you don't like bloated bass and veiled mids, it will be your best friend. Included tips are great if you can get a good fit. For me they're too short. I also don't fancy the comply tips with the UE900 as it takes away a bit of the treble extension from an iem with already smooth treble. Sonically I think that the small auvios are the best but fit wise the meelec balanced tips are in a league of their own. They are comfortable enough for me and once I have them in, they stay there but be prepared to do some tip rolling.
 
Sound: The stock UE900 I would describe as warm, bloated bass, veiled mids, slightly recessed but smooth treble. That's according to my preferences. But overall a good tuning for mainstream music. Very consumer friendly. The good part begins here. 
 
With (partially) blocked pinholes and 100 Ohm adapter: The sound is mostly neutral but on the laid back and warm side. The bass is solid and not bloated or boomy. May not be enough for some. It's decent in speed and texture. Faster than GR07 but slower than something like R-50. There is a healthy amount of sub bass but less than both GR07 and R-50 but more than Re0. There is no bleed into the mids apparent to my ears. 
 
The mids are smooth but still retain a good bite in electric guitars. They are pretty relaxed and a bit warm. Again I'd place them in between GR07 and R-50. Some might say that there's not enough edge to violins, higher pitched piano notes etc. I'd say that it's just right. Not dark, not bright, not grainy or edgy, just smooth but still well defined.
 
The treble is... smooth (surprise!) and well extended. It rarely exhibits any sibilance even in a bit harsher recordings. So a good balance and no glaring peaks. It lacks the air and presence that the Re0 has but it's way easier to listen to. 
 
Imaging and soundstage is decent at the price. It's not the biggest sounding. I suspect that it actually sounds bigger without the adapter. But for me the bloat makes it a big no-no. A quality that the UE900 has is to place two instruments at almost the exact same spot but still separate them well. Maybe it's my ears that are untrained but I find it intriguing. The layering is decent at the price as well. It looses out to ASG-2 (especially if EQ'ed) which beats it in both soundstage size and separation.
 
Out of BH(2) Output 2: Now this is where it gets really interesting. Compared to the stock UE900 it's almost a u-turn. The treble becomes more present, the extension increases. The soundstage gets more hemispheric or circular. Within that sphere the layering is excellent. Then there's like an elliptical orbit of sounds. It's bigger than the more spherical inner circle. In between these two there is little to no layering. It's a bit like electron shells. (The simplified version.) The sound gets a bit colder and more analytical, the bass body disappears slightly compared to only 100 Ohm adapter. Now the good part is. If you own a BH(2) you don't have to block the pinholes which should bring the bass impact to the level of blocked pinholes and 100 Ohm adapter. The BH pretty much takes away the bloat and veiling without blocking the pinhole. This is a more aggressive tuning but the way the treble is presented is to my ears almost flawless. Detailed and with great presence in the upper treble but without the harshness.
 
My ratings:
Build quality (adjusted for price): 3/5
Handling (fit): 3/5
Comfort: 4/5
 
Stock sound: ~8/10
With 100 Ohm adapter: ~9,3/10
BH(2) Output2: ~9,4/10
 
The best quality that the UE900 has with blocked pinholes and 100 Ohm adapter is that it just sounds good with anything you throw at it. It's not extremely picky with recordings. It's not a sibilance monster. It's not a bass monster. It's just a good iem that lets you listen to any music you want to. It doesn't present it in any unique way like ASG-2 or CK100Pro do. For me as a Swede. It's just "lagom" (which means not too little, not too much, with an undertone of this is the perfect amount).

Summary: These require deep insertion. Once inserted they are neutral, extended and adaptable. They don't really have any sound signature that shines through recordings but instead adapts to what is being played. Great for the price. Very versatile. 
 
I bought these used from a fellow head-fier. Coming straight from the GR07, they sounded bright, dry, flat, grainy and unrefined. However, just 2-3 songs into the audition I forgot all about them. The brain is great at getting used to small flaws. After that I just heard the great sound. They really show you what's in the music. If the music has plenty of mid bass, they play it for you, if the recording is cold and sibilant, so are they. A great iem at doing what they are told to do. At first there was one thing really bothering me, sibilance. After really pushing them in, it disappeared. I previously tried with some paper pushed between the nozzle and the tip to absorb the treble. That muddied up the mids a bit, killed the extension and increased the bass (something I think many would like actually) but for me, they lost their real pros.
 
I'm using them with the orange hybrid clones, a very deep insertion (for me not used to it) is required to get rid of the sibilance. The cables are a pain in the ass really but with some duct tape or similar it's fixable. Otherwise they just fall out if you're moving too much without being careful about the cord. The form factor is pretty good with small housings that feel durable. Wearing over-ear is my preference even if the ends point in an odd direction. Wearing them down produces too much noise for my taste but if I'm not moving it's fine.
 
NOTE: I'm just writing this a bit casually. The sound impressions are mostly from memory but I just had to listen to them to confirm. Really enjoying them.
 
Bass: Extended, quite flat without the bass boost or mid bass hump. Texture is quite good but lacks the "organic" (in lack of a better word, that's how I always think of it) texture and decay of the GR07. Quite fast response and good impact. No bloat unless it's in the recording. What really surprised me was the ability to produce a warmer sound with more mid bass in some live recordings where I've previously thought "hey, there's some powerful impact in the bass here".
 
Mids: One word "neutral". I've always had a hard time describing mids. There is a good balance between the lower mids and the higher mids. They neither sound warm, nor do they sound cold. Though they can be anything if that's what they're told. (OMG did I just rhyme?) The resolution isn't the greatest compared to higher end sets like R-50, UE900, GR07 or Re0 but like I wrote before. You get used to it and soon it's not really any problem, I don't feel that there's anything lacking there. Detailing is good. Overall mids are great.
 
Treble: Here's where the real issue lies. If they're properly inserted there are only hints of sibilance, mainly in the "sssssssss"-department and some cymbals. Otherwise they're fine. Not emphasized, the treble just blends in with the rest of the sound. However, if they're not inserted properly. There is some eardrum cutting sibilance with every "s". Compared to better extended sets like 2/3 blocked UE900 out of a C&C BH output 2, R-50 or Re0 they sound less airy less sparkly. However the R-50 (stock tips, don't have comply yet) and Re0 (hybrids) are more offensive in other ways in the high mids/treble.
 
Insane price/performance. Really. Buy them.