Sennheiser HD660s is a new improved headphone, based on the good but aging HD650 (and the still good but old 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 improved clarity and a really clean, smooth midrange.
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 is showing up on plenty of portable and desktop amps. If you want xlr, just add the DDhifi XLR adaptor from accessories section.
The new iFi line of dacs and amps are great value and include a balanced amp to match with the HD660s & included 4.4mm balanced cable. So depending on the budget the ZEN Dac, ZEN Can, new iFi Signature and the Neo all have 4.4mm balanced out and are good matches for the HD660s.