Now now, I wouldn't say the HD600s are brighter. At least for the rev3, they're about on par. The HE400s definitely have more treble and more bass. I prefer the mids/vocals on my HD600s. It's not much less bass, but yeah the bass on the HE400s is definitely better. That said, I love my HD600s already because of their laid back but detailed presentation. My HE400s (rev3, gonna exchange for rev3.5) sound great with my Lyr. I wasn't a huge fan of them with my E17 but I think the Lyr has improved them considerably.
The rev3 HE400s aren't really representative of how they are intended to sound though, which is dark and smooth and laidback with just enough shimmer in the upper treble.