HE-300 is, IMO, pointless. It's a dynamic headphone, so you're not getting any of the planar magnetic goodness that is HiFiMan's upper-tier products (with the 400, 500, and 6).
The HE-400 is very fun and colored. The HE-500 is very neutral and refined. The HE-6 is basically a better, harder to drive version of the HE-500.
I don't own any of the units, so I can't comment on how hard any of them are to drive.
Well that's a bit of an fallacy in and of itself. Them being dynamic in this case needn't be a vice. For what they do, and what they aim to do, and the price that they're doing it with, they do quite nicely. In fact, they were just nice, period (mid-centric, lush and smooth). Doesn't seem to make sense to compare them with the other models above them.
Mostly agree with the other impressions on the planars though. What'd you run them through while demoing them? Nvm, you haven't heard them.
Edited by TwinQY - 1/27/13 at 12:03pm