Thanks for your comparison, sect44.
I'd like to make a few comments, though.
First, as noted already, I don't own, and never have heard, the CK100 Pros, but do own the FI-BA-SS which apparently seem to share a few sonic characteristics with the ATHs.
I'm generally reluctant to speak of genres for phones and, instead, often prefer to speak of recording and/or mastering quality. Also, and kind of related, I always find it wrong and rather amusing that people tend to recommend bass-heavy phones for bass-heavy music/genres — aren't we adding extra bass to an already bass-heavy genre? Funny we don't see people recommending bright-sounding phones for bright-sounding music.
Anyway, I find that the FA4s won't always be that suitable with some classical orchestral works, but it will be fine with others (again, this largely depends on recording/mastering type and chosen volume levels). However, on chamber music and solo works (violin, piano, etc.) I find they do extremely well. They also perform brilliantly with jazz.
The FA4's layering is superb and this allows for a sound that is not congested, something phones like the W4, UM3X, SE530 / SE535, the old CK100, and even the SD-2 can exhibit. This layering presents a very believable soundstage, the sound is full, with authority, but not excessive which might easily lead to fatigue after 20-30 mins—this is an IEM I find it remains consistently exciting and engaging.
Fischer Amps have tuned this IEM really, really well. This is not a musicians' monitor (though I guess it could still be used by some musicians, too) or a so-called reference type of IEM; this is a great IEM that presents music very realistically and sounds remarkably close to $1,000+ TOTL IEMs; ordinary, no-nonsense people—people who have not been contaminated by plenty of audiophile nonsense, BS and silly biases often seen in forums of this nature—would even choose the FA4s over some of those much more expensive IEMs — people who also love music and good sound (more likely people who love / appreciate music more than gear and fancy audiophile jargon).
If I personally could only have one IEM and could spend 500€-600€ max, today the FA-4 would be my number one choice, without a doubt: comfort is second to none, and SQ is absolutely brilliant—and it comes with a standard Westone-type removable cable, which only after FitEar's removable cables, are still my favourite (and even FitEar cables are more microphonic and unruly!).