I have tried/owned the following: PC37X, HD598, HD599, HD600, HD650, HD700, HD800, HD 6XX, HD 58x and MANY others from other brands. The 660S is overall, the best headphone I've heard. It has no flaws. The tuning is what I would consider as perfect as you can get for a dynamic driver. No harsh highs, no recessed mids, no overpowering bass. Amazing imaging. Comfortable as any other HD variant. Better looking than any other HD variant.
It may cost a bit more but it's completely worth it. Makes the 600/650 sound old and out of date. These have far better imaging, detail, clarity. They aren't veiled in least bit but don't approach being harsh either. I don't know how they did it.