The talk about the driver alone is always 'I sleep.' I believe this person meant the back of the 'driver' is similar-ish, as in this picture, it is just impossible to discern the diaphragm material (or the specific polymer blend), voice-coil material, acoustic impedance of the baffle, and, of course, the entire structure of the chassis and earpads playing a role on one's head.
I actually agree on this, it's not solely about driver or driver-type, but about implementation. The implementations across units that use the SYS platform seem very similar if the frequency response is anything to go by (I of course am willing to be wrong about this when I actually hear a HD 490 Pro unit, I've heard the others and my judgment stands).
Thanks for the link to Jermo's explanation of the driver differences and why the SYS platform is more commonly used.
To quote Jermo, "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." He speaks praise but doesn't speak about limitations, just saying it would "take over the brilliance of the KDH580" with seemingly no downsides. Doesn't sound like a reasonable way to frame things; in any meaningful change to a design, there will always be trade-offs.
The fact that they've dropped this SYS driver in the 660S into an almost identical sonic circumstance as the KDH 580 driver in the 650 (same baffle, pads, chassis, nearly the same rear grille which is proven to have minimal impact anyway) and have the tonal differences we can see across basically any measurement rig, we can conclude rather safely IMO that the differences are up to the characteristics of the driver: material, dimensions, voice coil, rear damping scheme etc. To many, HD 660S was a categorical downgrade from the HD 650. Clearly Sennheiser felt people's myriad complaints about less bass and treble were loud enough to go back to the drawing board, which is why 660S2 exists.
I'm not going to say that the SYS driver is categorically worse, as that was never my intention with the post in the first place. I also never called anything a "flop." Of course any headphone will find its adherents, but to reiterate clearly:
I simply don't see the point on repeatedly retreading territory that has seemingly been done enough (and at better prices) when what people want from Sennheiser seems to be rather clear to me—the excellent midrange and smooth treble they're known for with the 6 series, but with more bass extension.
I understand better than most that this isn't an easy thing to achieve, and indeed with the post from Jermo you've shared, it seems like the KDH580 makes this basically impossible. But that doesn't mean the compromise they've chosen to live with is one I agree was worth making.
It seems with their current direction that they've settled on a compromise that has painted them into a corner of a midrange tonality that is incongruent with what made thousands of people fall in love with Sennheiser's 600 series. Furthermore, they see fit to make it harder for people to actually try or buy the HD 600 and 650 in stores, instead either pushing their newest 660S variant or generating pro SKUs like 490 Pro and putting them in the spot on a store shelf that would otherwise be occupied by an HD 600 or 650.
In the 660S2's thread here on Head-fi, or the countless reddit r/headphones posts about the 660S2 (
example) you'll find way more positive views than in discord echo-chamber servers, so perhaps one's perception of a product being a win or flop can be skewed depending on the bubble you're in.
Simply exchanging one echo chamber for another, and frankly I take issue with painting them as if they are similarly discerning groups of people.
While I frequent all of these spaces, the "Discord bubble" is generally the one where listeners are more experienced (having actually heard a lot of the gear they're talking about) than a place like reddit. Additionally, they tend to care enough to better understand the benefits and limitations of audio measurements, whereas Head-Fi is largely antipathetic to measurements as a tool or topic of discussion.
These two things greatly influence ones ability to make an educated judgment, I'm sure you would agree. For that reason, I am fully confident in saying the bubble you complain about is on average the more discerning pool of consumers than either reddit or Head-Fi, which is why it's where I tend to spend the bulk of my time.