Sound:
I have always used my FA-4E XB with source devices with a low output impedance and the largest included silicone ear tips.
Tonality:
In short: Warm v-shape.
The quad-BA in-ear with three acoustic ways (2 woofers, 1 midrange driver, 1 tweeter) has a forward, warm bass range, followed by a midrange that is neither recessed nor forward, followed by some relaxation in the lower and mid treble, while a small emphasis on the upper treble with good super-treble extension above 10 kHz can be heard.
The FA-4E XB is not reluctant in the bass range and has a lift of about 11, 12 dB compared to my Etymotic ER-4S/the ER4SR, an in-ear tuned to be diffuse-neutral in the bass range. Thus, the FA-4E XB is bassier than most other multi-BA in-ears.
There's no sub-bass roll-off, which is why even very low tones are reproduced powerfully and with a "subwoofer effect".
The low-frequency boost of the Fischer Amp in-ears doe not concentrate exclusively on the pure bass range, however, but also radiates audibly into the fundamental range and only reaches a neutral level corresponding to the central midrange at around 850 Hz. Accordingly, the in-ear also sounds warm and full-bodied, and the midrange tends audibly into the warmer and fuller direction.
The FA-4E XB does not try to counteract the warmth of the fundamentals by emphasising the upper midrange, as is the case with many hybrid Asian in-ears. Instead, the in-ear has a more relaxed tuning in the lower and middle heights, which forgives even worse recordings and also reduces sibilance in the vocal range of a recording. This recessed lower treble is also a reason for the midrange warmth and some darkness.
However, between about 8 and 9 kHz, the in-ear's treble peaks narrowly, but neither sharp nor strong (it's just a bit above neutral in quantity). This emphasises cymbals and is a mandatory counteract to the warmth, else the in-ear would be lacking freshness.
As the in-ear's super treble range is also quite well extended, subtle glitter and air are also audible if the recording allows it.
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Depending on one's personal taste, the tonal tuning of the in-ear, especially in a quiet environment, can be a little too warmth- and bass-driven for music consumption - even the lower midrange can be a bit too forward when acoustic and electric basses as well as deep voices are present.
For stage monitoring (e.g. for bassists or drummers) and portable use, however, such a tuning can be exactly the right thing.
Masking effects caused by foot steps while walking or ambient noises make the low frequencies of in-ears and headphones appear weaker, as long as one doesn't "fight" them by increasing the volume (which is never a good idea). A tuning like the one found in the FA-4E XB can counteract this very well.
For me, the FA-4E XB has proven to be a very good in-ear for occasional outdoor use - the sometimes already somewhat too present fullness then turn into a harmonious overall picture with subjectively only very mild, pleasant warmth, as the masking effects are bypassed.
Resolution:
The FA-4E XB definitely lives up to its price and the expectations of a good quad-BA in-ear and plays at a high technical level, although a little behind my Westone W4R and Audio Technica ATH-IM03 in terms of overall resolution.
The treble of the in-ear is well separated, comes right to the point, and shows no signs of bad definition. Individual notes are neatly separated and there is no "ringing"/reverb.
The bass range of the in-ear is, due to the rear ventilation of the woofers as well as the bass intensity and tonal tuning, audibly more on the softer and somewhat slower side for Balanced Armature standards. However, the FA-4E XB plays a good deal more controlled and precise than most in-ears with dynamic driver woofers.
Due to the good resolution and control in the bass range, the in-ear doesn't appear spongy or overwhelmed even with more complex recordings and music pieces.
The speech intelligibility and resolution of the mid-range is good - compared to some in-ears with "only" two acoustic paths, the FA-4E XB has a resolution advantage, for example the in-ear also has a higher resolution and sounds finer than the comparably tuned FA-3E XB (not to be confused with the FA-3E), whose mid-range produces less detail in comparison. Nevertheless, due to the tuning, one has to expect some masking effects of the lower midrange due to the powerful bass and fundamental range.
Soundstage:
Even though the FA-4E XB isn't necessarily the most spatially convincing and precise in-ear in its price range in terms of spatial imaging (my Audio Technica ATH-IM03, InEar StageDiver SD-2 and Pai Audio MR3 offer the more precise, three-dimensionally convincing and authentic stages), it doesn't sound unpleasantly flat or constricted at all, but has a stage that is relatively average to slightly larger and fairly open, wherefore it is quite believable, although not completely impressive or large.
Also, to my ears, it is rather round, with just a little more width than depth.
Hereby it resembles my FA-3E, which also sounds more convincing than some of its multi-BA competitors, but doesn't come close to the "elite".
Instrument separation and location are not negatively noticeable and are quite precise, although a small "fogginess" is present on rare occasions and the exact representation of "emptiness" is only hinted at, but not completely executed.
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Compared to other Multi-BA In-Ears:
Logitech/Ultimate Ears UE900/900(S):
The FA-4E XB is the noticeably bassier in-ear and also has the fuller sounding lower fundamentals. Vocals are a bit darker on the UE but warmer on the Fischer Amps in-ear. In the upper treble, the Fischer Amps in-ear sounds a little brighter and more energetic than the UE.
The Fischer Amps FA-4E XB is a bit softer in the bass than the Logitech/Ultimate Ears UE900/UE900(S), which sounds a bit less detailed in the midrange than the FA-4E XB. The treble, on the other hand, is level.
The UE has the slightly wider soundstage, while the FA-4E XB's has noticeably more spatial depth, which makes it appear a little more authentic in spatial terms. Layering is also better on the FA-4E XB, however in terms of instrument separation, the UE900/UE900(S) is superior to the FA-4E XB.
Custom Art Ei.3 UIEM:
The Fischer Amps has the more present bass and fundamentals, which unfortunately also radiate into the lower midrange, so voices sound warm and a bit dark. In the upper treble, it is also the brighter of the two in-ears and emphasises cymbals more strongly.
I would say that both of them have a comparable resolution in the midrange. The FA-4E XB sounds a bit better separated in the high frequencies, while the bass of the Ei.3 is a bit less soft and faster and also sounds more detailed to me.
While the FA-4E XB has a really solid stage, it has always been a bit behind that of some very good in-ears in this range (e.g. compared to the my SD-2 and ATH-IM03). To my ears, the stage of the Ei.3 is bigger and has the more precise instrument placement and separation.