The fast answer is: the driver in the HD 660S and the HD 700 is the same.
The long answer: The older KDH 580 developed for the HD 580 and still found in the HD 600 and HD 650 is to this day mainly hand assembled, which is not ideal for some precision processes, even though it's extraordinarily consistent for what it is. It was optimized for minimal partial vibrations, which resulted in the extremely smooth and clear response which is for me still practically the optimum.
The SYS38 platform was created in the early 2000s to take some of the properties of the KDH 580 into a more affordable space. It shares the same basic geometry in terms of diaphragm and coil diameter, but is fully automated, comparatively slim so that it can be implemented in a variety of headphones and is highly variable in performance depending on the coil or diaphragm used. Since then it has been used in many 500 series headphones, in some PRO headphones like HD 280 PRO, and in our highest performance Bluetooth models like the Momentum 3 and recently Momentum 4. The highest performance version of it sits in the HD 560S, with the new polymer blend diaphragm and a very light aluminium voice coil. This transducer is also implemented without changes in the NDH 30. However, Neumann is incredibly strict on tolerances for this model in a way that it's on the edge of what's possible in headphone series production, which may contribute to positive characteristics like soundstage.
Coming back to your question, the SYS40 platform was originally devised as a peak-performance automated assembly driver to finally surpass the old KDH 580. While our transducers are generally devised to sound good "out of the box" and in many different acoustic systems, the SYS40 has originally been developed for the HD 650 enclosure. This is because we know that it is fairly optimal in aiding a natural listening experience. The advantages compared to our previous platforms are a stronger magnet, a vented magnetic yoke which reduces air turbulence during incursion into the magnet, a very consistent damping concept and lastly an undampened magnet hole. The magnet hole in the KDH 580 is heavily dampened and is drawn out (internally we call it Rüssel, ie. an elephants trunk), so it acts like a bass tube and slightly lifts up the amplitude in exchange for less "punch". So the new transducer would take over the brilliance of the KDH 580, but improve on bass impact and have overall less distortion. When the SYS40 was finished, the plan was made that the implementation should happen in something that visually connects the HD 800 and HD 600 series, in a design that would become the HD 700. While the design had some benefits such as fantastic comfort, overall it was not ideal acoustically, and we all know the end of that story. the development of the HD 660S was basically just putting the transducer into the headphone it was made for.
I don't know all the ideas behind the initial parameters, but they had some requirements like lower impedance for better drivability across devices. With the HD 660S2 we have optimized all parameters for what we think is ideal music playback, ie. much better detail retrieval and more impactful bass.
