The clarity that you're mentioning is what's driving to get the H6. I'm not a heavy bass enthusiast (I prefer neutral sound) so it should be a real pleasure to use. And by the way, how much burn-in did your H6 & DX90 get ? I will also be waiting for your impressions on the new firmware :D
Although I don't really believe in electronic parts burn in I just let the DX90 run when I didn't use it so it stacked up roughly 370 hours by now. I connected the H6 for the last nights and it's on 48h now.
I can't say I hear much of a difference, maybe because I don't remember in detail how the DX90 sounded in the beginning. The guy from the shop and some reviews I've read here claim that the H6 will improve with burn in.
The H6 (and to some extent the NAD VISO HP50) deliver what I was looking for in the very beginning when I bought my B&W P5: A portable version of my home speaker's sound. I don't know why today's headphones are mostly so tame in high frequencies.
Maybe it's because kids today listen at drum bursting volumes to get the Dre Bass and will find highs at that level painful and would return the headphones?