The drivers of the HD700 sit at the bottom of some things that resemble tea cups, so the soundstange of the hd660s has to be similar to HD650, not hd700, since they share the same structure. A fourth person says the HD660s have sparkle in the treble. I'm sure it's nothing like the HD700/800 or Beyers, but I was looking for just a bit more, not for something to pierce my eardrums. I can't wait to try them, but aren't available in Romania yet. From the way things appear to be, it seems like the HD660s will be a point of reference stronger than HD650. The HD660s seem to have tweaked the model into near-perfection with just a bit of sparkle and clarity.