Hi pietcux. To be honest I'm not really sure what you mean by 'technically more advanced'. If you mean details such as a frequency response expanded from 10 Hz - 39.5 kHz to 10 Hz - 41 kHz and a THD improvement from 0.05% to 0.04% and an increased SPL from 103 dB at 1 kHz / 1 Vrms to 104 dB at 1 kHz / 1 Vrms (source B&H) then I don't see the relevance. None of those technical improvements are audible. Yes, they all add up, and it's a different, theoretically better, driver, but at this level it just becomes a matter of opinion and taste rather than a case of one being clearly better than another. Indeed, you will recall that recently in this thread someone has stated that they prefer their HD 660 S to their HD 880 S! The technical difference between those two is far great than it is between the HD 650 and the HD 660 S - in favour of the HD 800 S!
Bear in mind also that you always pay a premium for a new product. 'The latest thing' is always going to carry a premium compared to an older product and invariably there will be a price correction over a period of time as the new flavour become not so new anymore.
But, that would be irrelevant if I really wanted the HD 660 S. I just still have a few problems with pulling the trigger (at the current prices). I've stated some of these before (such as doubts about the tonal correctness when it comes to wind and string instruments) but in addition, I'm almost totally satisfied with my HD 650 HDVD 800 combo. By all accounts the HD 660 S sounds better (or at least more exciting) and is easier to drive with lesser equipment, and I'm sure that they would sound fantastic on the HDVD 800, but the difference between the two on that amp isn't going to be big. The other slightly irritating thing is that, having paid a premium for the HD 660 S, I still wouldn't be a able to plug it into the HDVD 800 in balanced mode without buying (or making) a three-pin balanced cable for it, or continually swopping the balanced cable from the HD 650 (not something I'm prepared to do). That's just plain annoying........