Hi Ken---
agreed the 600s are fine sounding, but:
a) they will not work well at all with portable devices (or most PC uses) unless you want to tote an amplifier around, which I do not. The 590 gives you a goodly portion of the 600s clarity, probably a bit more comfort (IMO) and can be driven much more easily. And:
b) the 590 has a more aggressive sound, putting the listener more in the middle of the soundstage a la Grado. Most 580/600 fans tend to not like the Grado sound, but those of us that like Grado's up front sound but dislike their relative lack of neutrality can find a decent compromise between the 325 and the 600 in the 590...
That said, the 600 is undoubtedly the finer headphone, but I think the 590 will continue to serve its own niche of users.
As for Senn's marketing and design, they deliberately manufacture more phone models than can logically be supported in hopes that some will serve a larger propulation of users than others. The ones that do survive and the rest get tossed (545, 575). This is exactly opposite of what Grado does, but it seems to work for Senn. Problem comes when they come up with a crop of crap like the period where the 500/570/400/433/490 ( and arguably the 495) were all in mainstream production. Fortunately for Senn and us, they've moved far beyond that stage with the newer 2x2sm 4x7s, 280...and the 590!