Despite it's ridiculous amp requirements, I feel the HE-4 ($450) was like the planar version alternative of the Pro 2900, with a meatier sound, better bass presence, and not as metallic treble. They both have sparkly treble and recessed mids, but the HE-4's is more rounded/fleshed out on both, and doesn't have that weird S-logic presentation.
I'd still get the HE400 with velours, which is better than both (slightly over the HE-4 which is more v shaped), and a lot easier to power.
That being said, I need to get my hands on another HE-4 to directly compare to the HE400 and see how it fares side-by-side. Typically, I'd prefer the HE-4's frequency curve over the HE400's more balanced approach, but I'm in utter love with the HE400, so I wonder if the HE-4 is as good as I remembered. I just wish it wasn't so hard to drive. If Hifiman brought out a newer HE that sounded like the HE-4 with easier powering requirements, I think I'd have to try them out.
Edited by Mad Lust Envy - 8/4/12 at 1:28pm