@Torq, @taronlissimore and @generic have provided great reviews and impressions that appear to be for the most part in agreement with each other and with my impressions as well. There is nothing of significance for me to add, just a few trivial comments.
I would say that the AFO in comparison to the AFC are rather creamy sounding and that once you adjust to the sound of the AFO they can be very enjoyable, especially for rock, pop and recordings that require more forgiving gear. As others have pointed out, paradoxically I found the AFC to be more closed, claustrophobic and in-your-head than the AFC, which is the main reason I sent them back. Otherwise they could have been a rather fun headphone to own.
Regarding the perception that the AFC are lifeless, I do agree that they require more juice to sound their best then one would assume based on specs, which put me at odds as to whether or not I should keep them as a portable headphone because they are so light and comfortable to wear as well as being very good at noise isolation. They are also a very sturdy headphone and do not seem prone to scratching or to being easily damaged. They sound fine out of my LG phone at almost full volume but properly amped they sound really great, especially out my Phonitor x. On other amps, such as the THX 789 I find that the Focal Clear are quite a bit more dynamic sounding and in comparison the AFC do sound flat but that difference is smaller out of the Phonitor. I assumed that it was due to the Phonitor’s 120 V rails but @Torq identifies current as the juice the AFC need.
A friend of mine uses the AFC on-the-go and gets the juice he needs out of a Vorzuge Duo II. He says it does the job very well.
When the Focal Elear first came out I was immediately impressed with how dynamic and punchy they were but after a bit of time I found that I had to constantly adjust the volume as on some passages I really felt as though they could punch a hole in my head. When the Clear came out, I found their dynamic energy to induce the same exhilaration but with just the right amount of politeness so as to not feel the need to constantly have my hand on the volume control. For me, the dynamic energy of this line of Focal headphones were a clear departure from any other headphone I had heard. In comparison the AFC are somewhat flat sounding and lack the sharper and more clearly defined transient edges of the Clear but that also makes them a more relaxing listen and in my view more appropriate for background music listening while working, whereas the Clear demand too much of your attention.
@generic mentioned that he found the Elex to be brighter and more brassy up top but that is not what I hear with the Clear, although being over 50 and having been exposed to a lot of loud music I can’t hear anything above15K.
Because the AFC are great at isolation, lighter and more comfortable than the Clear and have a texture that in comparison is smoother and more relaxing, while also being technically almost as competent, I find them to be a great compliment to the Clear and would recommend them as a sealed/closed-back addition to the Clear/Elex.
One note on the question of the filters; the AFO came with, I think, 4 different kinds but my AFC only came with the black open-cell foam filters, which I prefer to having none. In his review @Torq said he preferred the AFC with the white 2 notch filters. Were those the ones that came with the AFO or have others received them with their AFC? I think the 2 notch provide the most damping so I am curious as to why @Torq preferred them.