You're stating that as definitive fact, but it's opinion. I understand you find it pitch perfect, but to me it sounds a bit off, and sounds more lifelike on the HD600. There's no right or wrong, just preferences and opinions. I respect your opinion, I just don't like being told my opinion is wrong.
Its been no secret that many enjoy the midrange on the 580,600,650 ... for its lush and romantic presentation. I would argue the older 580s are a little less "romantic" and slightly overall thinner (relatively speakering) though still warm'ish.
When comparing to some mid centric focused headphone like dt48A (5ohm) , dt48E (25ohm), dt100 (16 and 2000 ohm variant), dt480 (25 and 200 ohm variant), as these were used for voicing in televison and movie sets, it is apparent most of the 6 sennheiser is "enhanced"
Natural and Romantic were both used almost simultaneously in this 6 series description for decades.
To me, when comparing a very flat midrange with no "character" or "soul" or lack of musicality, you can really hear the voices in a different way.
Now, to me, both are not "off" but they are distinctively different.
After all, to romanticize, by definition, is to deviate from reality to a certain degree.
That is not to say, the 660S is "off" or the 600 is "on", they are both not "off" , but I would say 660S seems less enhanced/romantic. Which can be more boring and less intimate I will admit.
It is nice to get the goosepimples with the 6 series midrange magic when it's there, and also nice to hear slightly less enhanced but still somewhat accurate voicing with a few less goosepimples
660S1/S2, and i did hear them side by side with the questyle cma 12 were less "intimate" , but the singers I know didn't seem too "off" . I wouldn't say it was the BEST in terms of voicing (all other things aside) for me personally, but it is still very good.