Sennheiser HD580/600/650 got a rep as hard to drive cans, but they're really not that bad. They're highly efficient (for full-sized open cans), which is good. Then there's the 300-ohms, which is both good (presents an easy load; doesn't demand a lot of current) and bad (your source+headamp must have enough net gain to get where you want to go volume-wise). On the other hand, something like a Grado won't require as much voltage to go loud (because their lower impedance actually drws more current & power form the amp at the same volume), if you can stomach their sound :)
The other reason they got that rep is that they do "scale" their performance with higher-end upstream gear. However, that's more a relic of earlier head-fi days when there were few other high-end headphone options besides Stax. These days, the HD600 series still represents a GREAT value, but they have a low (compared to the best) and hard glass ceiling on their performance, so it's not worth dumping much money into a dedicated HD600/650 system before you should explore other options.
