I don't think these necessary beat MDR-V6 completely. MDR-V6 are hotter and more aggressive, but detailed enough. But these 668B are so accurate headphones that music sounds so natural that I am listening in tears, full of emotion. I had to add a touch of bass with EQ, just slightly so that there is that sensation of bass. These extend very well into 20 Hz but the bass isn't exaggerated by default.
The soundstage is very accurate, these headphones give all that there is in source file. I use 320 kbit/s MP3 or FLACs. If there is distortion in music these phones distort too. So I need to pick songs that are good quality.
At first the cheap plastic repulsed me and I didn't like to put them on my head. But now that I have used to the great sound the build doesn't bother me anymore.
I would like to build a shielded cable for them. It seems to take quite a while to get to know the sound signature of these headphones, it is so neutral.
If any of you know Head-direct RE2 that have reputation of being very accurate earphones, 668B have pretty similar detail but sound much bigger and better.
MDR-V6 suffers from a little harsh highs and closed soundstage, when compared to 668B.