HE-500 is good most of the time and anybody plugging it into anything will get it's "proper" sound.
HE-6 can put the hurtin' on you if you don't give it enough current, dem treble spikes.
Taming something difficult is fun though, and thats why it has something of a cult following. I personally believe it's in my top 3 headphones, along with HD-800, and LCD-2.
Which, the HE-6, or the HE-500?