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.
As much as I’d like these to be an upgrade it’s possibly they aren’t one when both are driven properly because honestly the HD 700(when corrected) isn’t really much of an upgrade over the HD 600/650 in terms of overall driver fidelity. It’s possible the drivers are essentially retuned HD 700 but I could be wrong. You don’t get a truly notable jump in driver fidelity from Sennheiser until you get the HD 800. It’s why I’m holding back and waiting on numerous reviews to see if it’s actually a better headphone or not. If it ends up being like how you say it could be I may just get one of the older siblings as my system is better suited for high impedance headphones. I’m expecting it to be a more efficient version of the HD 650 that does some things better but maybe not everything. The impedance difference will definitely change amp matching which will make it a little more difficult to compare.
The HD 600 vs HD 650 debate still ensues and this headphone will likely just add another headphone to debate getting alongside the HD 600/650. Actually wouldn’t mind a little more vocal range and midrange presence over the HD 650 more akin to what is found on the HD 600 as I feel the HD 650 weakness is vocals for me as it fails to grab me like the HD 600s vocals can but the HD 600 doesn’t seem to be able to create the same euphony the HD 650 can. I am really hoping the HD 660S can find that much desired middle-ground between the HD 600/650 that has been long requested.
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