Okay, time for sound quality impressions:
For starters, I think the high end is the most interesting/unusual aspect of the FA-011. I haven't heard anything quite like it (recall my limited experience though), and it's almost as if the headphones respond to treble in a totally nonlinear way.
On the one hand, the high frequencies are strongly, STRONGLY emphasized, and by that, I mean that it's not subtle to me at all:
For one thing, the background treble details (intentional and unintentional
) are put into sharp relief, and the louder high-frequency sounds have an obvious bite to them. If you have a bunch of low-level high-frequency garbage in a badly recorded or badly compressed track, you're likely to hear it and be distracted by it far more than you might with most headphones. The energetic treble can also be harsh at times, and there are some songs where it's a bit much for my ears.
On the other hand, I keep expecting the hardcore treble emphasis to overwhelm the mids with uncontrolled screeching sounds and racket...and it's really not doing that at all. It could be my imagination, but the loudest treble sounds don't seem to be magnified nearly as much as the [otherwise] subtle and modest ones. This is what I was referring to with my "nonlinearity" comment: There's a ton of high-frequency energy, but the extra emphasis seems to be reduced for high-frequency sounds of higher volume.
Technically, I think the mids are a bit quieter than the boosted treble and mid-bass, but they're crystal clear and well-separated, and their placement in the excellent soundstage highlights them in a different way. Every time I'm tempted to peg the mids as recessed, the FA-011's surprise me by putting them in the spotlight again. The mids are well-damped and crisp as well, and it sounds like every bit of mid-spectrum energy I'm getting is coming directly from the driver instead of from echoes or resonances. That might be par for the course for open headphones, but these are the first open pair I've ever owned...and I'm pleasantly surprised at the directness of the sound. There's a profound sense of airiness, and I think the highs are also carving out space around the mids and giving them more distinct boundaries than I'm used to.
It's interesting comparing these to my Re-ZERO's: They're probably about equal in terms of resolution, but the details pop out more on the FA-011's due to the superior imaging and stronger highs. The Re-ZERO's have a far more even balance of mids and treble, and it gives them smoother, subtler, and more natural overall tone than the FA-011's, which sound a bit thin. However, the FA-011's soundstage and imaging are just leagues ahead of the Re-ZERO's (or the cheaper HTF600's) though, which also imparts a more natural quality to listening. Unlike the Re-ZERO's, the FA-011's also have the bass to match...
...and speaking of the bass, it's tons of fun. Now, my HTF600's have been called basshead cans with almost as much emphasis as the XB500's, and I originally bought them to get an idea of what bassy headphones are actually like. I really like them overall despite the veiled highs, but the bass didn't floor me the way I hoped: The HTF600's have plenty of bass presence, but it's much more balanced with the mids than I expected going in. They were supposed to be my "guilty pleasure" cans, and I came out thinking the bass emphasis was pretty restrained. I thought, "If these are nearly as bassy as headphones get, I might need to stick with speakers on my serious basshead days."
In contrast, the bass on the FA-011's is really impressing me. It hits noticeably harder, faster, deeper, and "closer" than my HTF600's*, and it's nicely textured. There's a big "mid-bass hump," but I think it's actually centered in the middle of the bass range instead of the typical 100-120Hz area between the bass and the mids, which would account for the overall deeper sound/feel. Even though the bass is strongly pronounced, I haven't noticed it intruding into the mids at all (or at least not vocals). The bass doesn't impose itself where it's not called for, but it can and will be quite powerful if the track is bassy, and it punches me in the face when I deserve it.
Unlike the HTF600's, the FA-011's are bassy enough to qualify as a true guilty pleasure for me.*
[*Update: It turns out my impressions of the HTF600's bass were tainted by the common channel imbalance issue, which affects bass the most and keeps popping up whenever I let my guard down. According to a comparison I made this morning, the HTF600's at their best have at least as much deep sub-bass 60Hz and below and a little more at 30Hz (that said, cranking up the volume for a 30Hz tone tends to make them sputter and break again for me though). That's roughly volume-matched by ear with a 1kHz tone. I'm now duly impressed by the bass emphasis on both the FA-011's and the HTF600's...even though it's hard for me to get the HTF600 working at its full potential. The FA-011 seems much better at avoiding bass bleeding though.]
The bass decay also sounds natural but fast to me out of an O2. I'm not sure if I'm hearing something different from Gmork or if he's just more discerning, but I'm not hearing any bloat or boominess to speak of. (I'm definitely not hearing any muddiness or lack of clarity whatsoever.) Granted, the bass CAN be bloated if that's what's in the source, but I'm judging the FA-011's bass capabilities based on the fastest decay I hear rather than the slowest. Our contradictory impressions might be due to amping differences, but the bass quality and overall quality is still pretty good straight out of my laptop without an amp, to be honest. Then again, my particular laptop's headphone output might be a bit of an exception to the rule: The bass does sound boomier straight out of my iPod Nano, and the overall sound becomes a bit congested that way too. (All of that could be my expectation bias talking, but I don't have the means to do an ABX test.)
Overall, I have to say that I love the way my FA-011's sound. Like LFF said, these headphones are NOT neutral, and the hot treble definitely isn't for everyone...but man, they're loads of fun. I've been enjoying music today that hasn't held my interest in a long time, too. There's just something addicting about the sound of these. (As a completely random side note, I need to watch out for listening too loud: I usually listen at very low volumes, but I've been listening to these a bit louder today. I don't know if I'm indulging myself, or if these headphones really just sound better a bit louder. I really wish I had an SPL meter.)
My biggest complaint has to be the comfort: My ears didn't get hot like I expected, but the shallow pads and clamping force are problematic for long-term listening. My ears are hurting right now, and I just can't listen anymore for a while...no matter how much I want to. I'll probably stuff the pads with paper towels to alleviate this problem, but I can only hope it won't detract from the sound.
Speaking of mods, I ended up making my foam cutouts a bit bigger in the process of making them more symmetrical. They now expose two-and-a-half rows of holes. (Actually, the right one still isn't a perfect circle, but it'll do.)