As I noted earlier I like some headphone that Tyll doesn't like / hates like the HD700, most Grado, and Fostex TH series. There are headphones that Tyll likes that I don't care for like the Ether, EtherC and Ether Flow, HD600, HE1000, X2 and HE400S.@DavidA - would really appreciate if you would elaborate on where you agree and disagree with Tyll. Thanks.
With the HD660S I'd have to agree with Tyll that its not a replacement for the HD650 since its closer in sound to the HD600 and should have been considered a totally new model in the HD6XX line (or considered a replacement fro the HD600) like how they added the HD650 to the HD600 to give 2 different tunings of the basic six hundred series. I'm one that had some issues with the HD600 sounding harsh / sibilant with some tracks due to the small treble peak and the HD660S didn't have this peak so to me its a good replacement for the HD600 while the HD650 is just a smoother more musical sound IMO but I think the supporting gear that one has will affect what an individual will hear.
When I went to a friends house to listen to the HD660S I took my HD650 and HD6XX and we compared them on various amps (MicroZOTL2, EC ZDS, Elise, BH Crack, HA-501, Liquid Carbon, Magni3) one thing we noted was that the HD660S sounds pretty good on almost any amp while the HD650 did sound better on the tube amps to both of us and my friends 2 daughters who are both musicians (violin, piano). The HD700 should not be considered within the six hundred series since its just a different sound signature. These days I'd rather listen with my original Ypsilon build which is what I replaced my HD600, its very easy to drive (sounds good even out of my crappy phone) and the sound is more dynamic, better bass and smoother highs with only the mids of the HD600 slightly better to me. My HD650 has been replaced with a Ypsilon R1 which is also very easy to drive, more dynamic, bass that goes deeper, cleaner and more detailed and highs that are more extended and smoother, it has made the HD650 sound boring to many after hearing the Ypsilon R1. The only down side to these Ypsilon builds is that they are DIY and not exactly cheap ($300-$450 for parts).
In the end for those that don't want to deal with amps and DACs the HD660S is a no brainer and a easy recommendation IMO but for those that are willing to carefully consider DACs and amps I'd still recommend the HD600/650.