Well the S680 are a direct upgrade to the S400/S500 models.
I had the V55 for about 2 years and they never missed a beat, great for mixing Trance music! I sold them onto a mate of mine in Cardiff and he loves them. For the price they are an amazing set of headphones imo.
I've been looking to get a V55.. And if the S680 is really a direct upgrade to the S500, I'll definitely look into getting those.. How do they compare with the 500? How are the pads? Is the bass as boomy and uncontrolled? I did the cork mod on mine because I couldn't stand how booming they were..