I own the HE-400i. I do not own the Senn HD 660 but i do own the Senn HD 598, which is a lesser quality headphone than the HE-400i for most smooth listening purposes. With that being said, I exclusively play video games with the Senn HD 598. They are brighter and louder un-amped, which adds to the game effects. They also have a wider sound stage than the unamped HE-400i. Amped, however, the 400i is almost impossible to beat for both music and gaming.