I wonder how they compare to the HD580 precisions?

From what I've read the 580s sound very similar to the 600s. They are pretty much the same headphone except Sennheiser spent more care matching the drivers on the 600. If you mod the grills on the 580 to the 600 or 650 grills. The sound is almost identical.

^^^thank you for that info I hope the build quality is substantially more rugged than the 580s which in my case have vibrated themselves into buzzy uselessness in just a few years of daily use.

The 600s have a more sturdy build than the 580s. The plastics used are less fragile. I've seen many old generation Sennheisers with cracks around the headband adjustment. Something that doesn't happen to newer cans. If you want the best quality plastic, the 650s are actually built better than the 600s. There are a few reviews where people have accidentally broken the 600 headband from over straining. This doesn't happen as much on the 650s. Hope this helps in your decision. I'm a diy sorta guy. If I was you I'd maybe look into buying the replacement parts of the hd600 for the headband and yoke assembly and migrate the drivers of the 580 across but that's just me being cheap haha.

if I was more mechanically apt i'd do that. the main problem with the 580s is something deep inside them starts rattling on the lower notes. sometimes if I grab tight around the frame the rattling lessens but not always.

That sounds like the housing around the driver is loose. The lower notes would most likely vibrate the driver more. You take them apart and watch them while you play music.