The HD-580's and 600's use the same drivers. You just get the Metal Grilles and Carbon Headband with the change in model. Pick up a pair of the 580's and pop in the 600 grilles, very very sweet sounding setup for the money. The 650's are slightly more laid back, a touch more forgiving but they're not near twice the can which is what you end up paying for them.
If you're going to invest in the Cardas cable you should consider moving to a balanced rig...
The 595's are a different lower impedance beast with a much more forward sound.