I realize that this is a HUGE thread, but you could have done a bit of searching before posting. Impressions regarding your question have been posted multiple times.
I personally use the DX90 with my Beyerdynamic DT-880/250-ohm and T1/600-ohm cans and feel no need or desire to use a separate amp. Others have posted the same regarding the Senn HD-800, AKG's and some of the Hifiman cans (including the OP).
Just beware that the DX90 is super detailed and airy on the top end (in a good way), but that might give you a clue as to which HP or IEMs would have good synergy, or not.