Just wanted to report back after picking up a pair of H6. The sound is much more balanced – it doesn't have the overwhelming bass and harshness that bothered me with the H9 (1st Gen). It's a more open sound than the very forward sound of the H9, though not quite as open and airy as my closed-back Beyers. The sound is really quite impressive for such small and lightweight headphones.
Side note: the cable that ships with the Beyerdynamic Lagoon fits the H6. It's a bit tighter fit than the stock cable, but it does fit. Funnily enough, I find the H6 sounds better with the Lagoon's cable – it has a bit more body, sounds a touch livelier and opens up a bit more. I was surprised how much it changed the sound (I thought it would sound about the same).