Hmmmm....decisions....decisions...
As I write this, I'm doing some final research before whipping out the credit card. I've found
this review at goodcans.com, which says that the Sennheiser HD-580 is being gradually discontinued and replaced with the HD-590. (But I can't find a date on the article, so I can't tell how old this information is.) It says there is no huge difference between these models, except for some minor variances in construction, and no difference in audible sound quality (according to the author of the review). Granted, this is a .com site, which (generally) means they are selling whichever product is their "favorite" as dictated by profit as much as by what offers real value. So as I'm reading it I'm keeping that part in mind.
Also, my eyes keep going to the HD-600. *sigh*
OK...prices...it's gotta be worth the money, right? 10% "better" doesn't justify 50% more money. Lowest prices I've seen on each of these, using etronics as my first reference (as I've heard they have a pretty good rep and really low prices)...
HD-580 - MSRP $259.95. etronics: "Our latest price on item Sennheiser HD-580 Circumaural Headphones is $157.00." Nice!
HD-590 - MSRP $269.95. etronics: "Our latest price on item Sennheiser HD-590 Prestige Dynamic Audophile Headphones with Bionetic Design is $163.89." Nice!!!
HD-600 - MSRP $449.95 (?). (:holding breath:) etronics: A button that says "Email me when this item is available."

Headroom (www.headphone.com) has them listed at $229.00, but out of stock.
So far I've only heard of marginal differences between all three models. Only physical design differences between the 580 and 590, with supposedly the same sound quality. The 590's have been reported as being slightly more comfortable and having a slightly better cord design. I've heard the 600's have a little better sound characteristics (or just better stats, perhaps?), but might be a little *less* comfortable than the 580 or 590 because they're somewhat heavier.
BOTTOM LINE: ...IMO, and considering that I'm ready to buy now and won't be waiting any longer...The price difference of $6.89 justifies the HD-590 over the HD-580. Especially considering that my *only* complaint about my Grado's is
comfort related, so I'm putting some more emphasis on comfort than other people might. However, even if I
could get them right now, the HD-600 at $65.11 more than the HD-590 is a difference I probably could
not justify. (And yes, I'm basing this partly on the fact that in Roberto's recent 64kbps listening test I rated perhaps half of the test groups at 5.0

, and therefore would likely not benefit from a marginal difference in sound quality from various high-end headphones.)
And with the money I save, maybe I'll grab a pair of MX-500's for $14 to replace my Sony MDR's. Etymotics?...heh, maybe next year when my personal "audio equipment overspending plan" starts over.
@PoisonDan: I looked and looked for the HD-535 to see if anyone had compared them with the 580/590/600, but couldn't find any reviews or comparisons. Sorry. Maybe someone else who owns a pair of them will jump into this thread.