Pedro, that's because detail brings out harshness, it's inevitable. That's why HD650 is so smooth and "silky" and "woolly" etc, because it masks harshness and detail in the recordings to a degree, that's why it's veiled. A headphone can be either veiled/smooth or harsh/detailed, it cannot be very smooth and very detailed. Or maybe you can find a balance. I'm sure HD660s aren't harsher than most Beyers, AKGs etc, it's just harsher than HD650....which is somewhat veiled, and veiled is not something bad if you want smoothness and relaxing sound and don't count every bit of detail.
I can't really agree, the most detailed headphones I've ever heard weren't harsh in the slightest, in fact some were some of the least fatiguing headphones I've ever put on my head. A bit overwhelming at times yes, but not what I call harsh.