Well, on my system (separate dac and amp with all cables upgraded, ~$3000) I don't find what you described to be the signature of the ADX. But on the other hand, I do believe they can sound like that on unmatching systems. These cans are like magnifiers into the system, surely are very hard to satisfy. Maybe this partially explains its market share.
Yes, I may suggest we are talking about DAC-AMP-HPhones matching this case, rather than about signature (or about matching which changes signature ).
Physically, a known high-frequency boost in the 5k's response curve likely produces the "dust" , as for me.
Frankly, before auditioning 5k I was wishing it to win HD800S, which didn't happen in my setup apperently....