I've compared the HD660S to the HD650 and they're similar but the difference is the HD660s doesn't boost the bass as much but as a result the 5k area is a little rougher, it doesn't sound as smooth in FR as the HD650 despite being similar. Another difference is on higher end amps the HD650 starts to pull away those HD700 drivers have their limitations. If anyone plans to stick to cheaper amps and dacs I feel they will get more use out of the HD660S than the HD650. On high end OTL and OTC amps I would stick with the HD650.