840C is a good current-production DAC; heard Morphsci's, it was solid. I like older stuff such as Parasound, very analog-sounding compared to what I've heard amongst current offerings. Haven't read good things about the DAC1 compared to more vintage-esque DAC's from the 90's.
HD580/HD600 > HD650 for my tastes. Even amped properly the bottom end on 650 was far too bloated and ruined any sort of balance. HD600 is pretty adequate for most genres, maybe lacking "speed" and a little anemic on the low end for rock/metal/electronica.
Cables are not worth the price of admission *blasphemy!*. I'd just stick with the stock HD650 cable (can be used on HD580 and HD600). It's a little more durable, less prone to tangling, seemed to function as a cable should.