https://en-us.sennheiser.com/headphones-audiophile-high-end-hd-660-s
Based off the the description of the item and the fact that these share the same price point as the HD650, I believe Sennheiser intends the HD660S to be a lower ohm version of the HD580/600/650 family, sharing the same celebrated sound signature. The transducer is "higher performing" because it can convey the same sound with lower coil sensitivity and hence allows for more listening use cases. Kinda like the Beyer DT 880's product strategy.
This would also explain why they needed to create a new driver.
If the HD660S is a strict improvement on the HD650 sound as some have suggested (i.e. same midrange, better imaging, less low end distortion), I doubt that it'd MSRP at the same price point.
What will likely happen is that the HD660S will be the HD6X0 that's best for a wider audience. The HD650 has had its long line of "great sound, but may not be for you" ilk of reviews due to its high impedance and source gear demands. Sennheiser has had much more reason to be bothered by that track record than the relatively stellar if not legendary reputation of the HD650's sonic performance. This wasn't a problem when a $500 MSRP pair of cans was considered TOTL in the headphone game a decade or two ago and hence could afford to be a niche product reserved to those with the dedication and enthusiasm to invest in and figure out source gear pairings. Today, smartphones abound, listeners are much more mobile, and the TOTL headphone prices have inflated by an order of magnitude. This means that $500 is about the "upper middle class" market segment, and the number of units one must move at a $500 price point to make sense from a product portfolio standpoint means the $500 product simply must have wider appeal.
This is evident in Sennheiser's frankly strange offerings like the
HD650 Apogee Groove that couples the HD650 with a USB DAC/AMP on-wire dongle. This is a transparent if not flawed attempt at fitting the HD650 to more modern use cases.
The HD660S is the better attempt in that direction.
In addition, these two different impedance levels will significantly alter the amp/can pairings of the two headphones. While the HD660S and the HD650 might share a very similar sound with a solid state, neutral, transparent amp, one will simply not be able to pair the HD660S with something like the Bottlehead Crack with its 120 ohm output impedance unless one likes extremely flabby, over-dampened, and distorted sounds. The HD650 will sound much better with high output impedance amps.