Everybody shits on HD660S, I think its best sounding out of the box headphone I ever tried, I absolutely love that frequency response!
In world of audio, there are two kinds of frequency response targets, fun and flat. Flat is for the profesionals, its meant for production work not for joy, atleast that is the common opinion. Then there is fun tunning, unlike flat, there is not just one target. The current fashionable fun target curve is the Harman curve and its derivates. Before rise of Harman, the popular curve was the V shape, boosted bass, boosted treble. There is quadrillion unique fun curves made for specific headphones. Each fun headphone with its own one of a kind fun curve.
Here is the thing, I fucking hate all the stupid fun curves, they all sound like shit. Except one, the HD660S. I think its masterpiece. Instead of another V shape or another Harman curve, Sennheiser made something very simple yet very original. They took the super flat response of HD600 and put gentle wide dip from 1 KHz to 5KHz. This simple modification has profound impact on the percieved sound.
1 - 5 KHz is the peak resonance gain of the ear, the shape of your ear boosts these frequencies strongly. Loud sounds mask other sounds that are close in spectrum, this is called frequency masking. By negating this ear gain, the sound falls much flatter on the eardrum. The result is that everything except this 1-5K area sounds louder and clearer becose the ear resonance is the limiting factor of how loud can you play before it starts being uncomfortable.
Its only headphone I ever tried that sounded so perfect out of the box that I had no desire to use EQ, its so fatigue free, pleasant and balanced, its joy to listen, HD650 in comparsion sounds bass bloated and with harsh sparkly treble, HD600 sounds neutral but kind of small and thin. Also, I dont know if pads are different but the soundstage was around 15% wider than HD600/650, I love wide soundstages.
