Only now that I see these graphs do I totally hear everything.  They sound just like this.  My source was just very concealing.

OK since we have measurements I'm going to post the subjective impressions I wrote up when I first heard these, before I opened them up, and which I never even got a chance to post because I was attacked by rabid fanboys for briefly pointing out an issue with the K550 in my even briefer first thoughts.  Note how positive I'm trying to be here!  I can move these elsewhere if anyone wants.  I just thought I'd juxtapose them with these measurements in light of the madness that unfolded from my impressions on HF.

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I put them on and my in initial two thoughts in this order were:
1. Wow these sound pretty fast and also fairly even
15 seconds later
2. Wow something is wrong with the upper mids
These were my initial impressions of the phones stock:


Pros

No treble emphasis, which was a surprise considering AKG's recent history.
midrange headed down to bass is pretty flat/even until it rolls off the bass, but it has enough extension to be enjoyable.  Bass is less than neutral though I'd say.  Decent punch.  Kind of HD600 like but of lesser quality (from distant memory of HD600)
Mids are not thin sounding, and sound natural unless/until they enter the glare zone.  But there is a natural richness, as well as air to vocals.  Good balance. 
Overall pretty even sounding with pretty good tonality, except for that spot in the upper mids that sticks out sorely and a general upward tilt to the FR, ie the "clarity FR"

Still, tonality is good enough that most instruments outside of that spot basically sound like they should, which is more than most headphones can say.
Overall they sound fairly fast-ish.

Cons

upper midrange glare.
Soundstage is decent but not great.  I think the glare really ruins what might be a spacious sounding closed headphone.  It's almost like there are 2 soundstages happening at once, the lower part of the spectrum sounds more spacious to me than the upper mids/treble, which sound constricted.  This gives them a lack of coherency, or a kind of bad crossover effect, where the sound is a bit disjointed.  I think this is because the resonance artifact in the upper mids destroys soundstage by getting in the way of the "trick" the headphones are playing on your perception. 
Not a treble head headphone.  But it's not bad by any means.  Treble is not that clean or fast sounding and seems a bit rolled off though not too bad.  A bit rough or textured or papery.  I'd describe the treble as "getting out of the way" (but not in a transparent way, rather in the sense that it doesn't stick out like many phones.)

Upper Midrange Issues
The upper midrange glare isn't as sharp as Grados or Ultrasones (from memory).  It's not as piercing or painful as many headphones though it might be described as "jarring".  It doesn't often cause me to reach for the volume knob in pain, but it does keep me from turning them up.  Any instrument in that range sticks out way too loud, and looses proper tonality as the sound in that range gets mangled by resonance and takes on a plastic tonality. You can clearly hear the coloration being added to what should be the original signal.  This is partly because it's such a specific problem spot. 

I put them on and my in initial two thoughts in this order were:
1. Wow these sound pretty fast and also fairly even
15 seconds later
2. Wow something is wrong with the upper mids

Almost exactly how I felt - but I jumped up and ripped the headphones from my head. I like to listen loudly sometimes though 85-93db.

I publicly guessed 6.5kHz on HF from 17 seconds listening at the SHOW. 5-6kHz is rarely bad, unless it's extreme, like the HD700.

I created this file to help people who are having trouble hearing the peak:

https://dl-web.dropbox.com/get/Public/peak.wav?w=ec9fd428

The first few seconds are regular pink noise, then with the main peak roughly and badly EQ'd out (leaving the other peaks intact, which raises tons of other problems).  It cycles back and forth a bit.  It's not perfect, but it helps you hear what exactly what the heck we're talking about.