Got the FA-003 today in the mail. From initial listening, I prefer the 8400 for a few reasons, but the Fischers are actually better than what I expected. Both work great out of a portable player. 

Give me a bit of time with them and I'll report back tomorrow :)

I owned both, and quite recently.  I found the Fischers to be the more comfortable of the two, until I modified the headband on the 8400s; then, it was a close call.  The 8400s are more compact, if that's an issue for you.  I ended up selling the 8400s.  I found the FA-003s to be superior in terms of clarity and bass impact.  I never tried either out of a portable music player, but I found the Fischers to be more sensitive coming from my uDAC-->receiver.

They are a bit more comfortable: The clamp seems to distribute some of the weight off the top of my head. Mid-range articulation is good, almost superb, but not better than the 8400. The Fischers seemed a bit mid-oriented, so certain instruments tended to hog the spotlight. I was sure to have heard something better, but then I switched back to the 8400 and everything was still there, even more accurately presented. To me, accuracy is pitch articulation in subtlest nuances. Close call though and quite similar in tonality, from what I can hear...

Treble is grainier/less clear on the FA-003. Some things are simply masked by a "veil" (don't want to use that term..) and transients blur. Tried listening to that percussion-rich piece from Jaco Pastorius' Birthday Concert -album, Reza, and while the FA-003 indeed had more impact on lower percussive instruments, some were not as distinct from each other as with the 8400. Same with the high notes (those quick doubles, whatever they are called technically, that Erskine does with the edge of the snare drum, for instance)

Also, I think the 8400 might even have more volume in bass relative to mid-range frequencies. Melodic bass, electric or acoustic, is slightly better textured (though quite good on the FA-003 as well), but NOT louder in volume on the Fischers. Absolute differences in pitch, however, are better distinct on the 8400. Listened to some War, and the somewhat simple lines can be tabbed with the FA-003, but they were a breeze with the 8400, however. Same goes to Jaco's soloing, except that it's a bit more musical, which makes the 8400 even more clear of a winner.

All in all, I don't think the FA-003's can be told to have better bass-impact, because it's not so self explanatory. Sometimes this extra thud, that came from mainly percussive instruments, seemed to hog away from mid-range clarity with the FA-003, but not distinctively. Sometimes mid-range frequencies were a bit muted on the 8400 due to the ever so slight boost in the treble. Mid-range dynamics, however, seemed to be better on the 8400: Differences in volume are more clear with the 8400 as they are with the 003. 

The FA-003 were a better headphone than I expected, but the somewhat monotonic (though tight and oddly, textured, and much better than the majority on the market) bass and slow treble transients made them a slightly more "boring" listen for me.

Similarly sensitive. All of my listening was done straight out of an iPod Classic 160gb (the three year old one, not the latest with those volume control stuff and the like), if that matters. Remarkably good, and much better than from my Zero dac/amp, which is not as clean.