Sennheiser HD660s is a new improved headphone, based on the good but aging HD650 (and hd580/600). The HD660s is easier to drive, better matching the more portable sources we are all using nowadays, with both a slightly higher sensitivity and a much lower impedance of 150 ohms. Sound quality is more open with better clarity and a really clean, smooth midrange.
*In store and now shipping*
HD660s are still light weight, at 260g, with deep velour pads for great comfort. There are now two cables included - one with 6.5mm plug & a 3.5mm adaptor. The second a new style 4.4mm balanced pentaconn connector that should see improving compatibility over time. (always nice to have more cables)