Sizzlers,
In my opinion, and playing in my "headphone" system (Cambridge Audio 740C cd player and Little Dot MkIII headphone amp), the AKG are highly revealing, highly transparent, and score very highly in the "technicalities" but they just can't set my feet tapping like the 668b's do, nor get me involved in the performance in the same way... They are kind of "cold", if you understand my point...
That doesn't mean the 668's are couloured, or all fireworks - they are not; indeed, they are VERY neutral and transparent, I dare to say detail and transparency are pretty close to the K701, but they manage to present it in a much more engaging way, and with punchier (not flabby, not wooly) bass. Highs are very extended, mids are in the exact measure, soundstage is great, PRaT is excellent. Besides, they accept any kind of music with equal aplomb.
The best thing I can say about them is that they have given rest to my AKG's, which have been lying in a drawer since I got the Superluxes properly ran-in - and that happened several months ago... Well, indeed, I ocasionally did plug them in a couple of times for 20-30 minutes, just to check if I had changed my mind, but they did soon returned to the drawer. The cheap Superluxes have something really special about them.
Cheers,
Filipe.