Fewtch: When one listens and then uses terms to describe it, the words are always some degree of metaphor or poetry. This is why I shy away from overused terms like "fast" or "extension"... better for us to use terms that are more poetic, and to use variety of wordings to paint a picture.
Listening to sine waves is virtually useless in assessing sound quality. There are about 5 things I could go into to show how rediculous your statements were, but no need now that you understand.
There are clear differences among the 580, 590, 600, and 650, and this is independent of break in or not. Sure, they change a lot, but I suspect the differences are larger than the breakin process differences. I've listened to all of them, broken in. I can't imagine listening to the 580s or 590s at any length. The 600s have a clear character which is beguiling but not very vivid in the midrange, a bit bright and harsh in the treble, and a bit warm (as opposed to sounding neutral, flat, and "extended") in the bass.
The 650s are a clear improvement on the 600s, unless you have a harsh source.
If you think they "use the same drivers" then you seem to have little understanding of their product development process. The Grados are far more like each other than are the Sennheisers. Drivers may have similar general design aspects, just as cars may use the same "platform." But they can be as different as a 1.8 liter VW Passat versus an Audi RS6 4.2L twin turbo, often said to have been built on the same "platform." No comparison, actually.
If you haven't heard the HD650s, then listen and believe those who are reporting on their sound. They are right (aside from lextek who for some reason had a bum pair or some system problem). You can't guess or judge without hearing them yourself.