Going from the HD650 to ZMF Eikon reminds me of going from a 5.25 inch bookshelf speaker to a larger floor standing speaker. Bigger sound, more substance. That makes sense and should result given the Eikon’s much heavier +2x weight and larger cup size.
I also observed that the Eikon is more sensitive, where it requires setting a lower volume level. Initially, this surprised me. But then I remembered the “pick 2 of 3” speaker building law.
The relative bass extension, sensitivity, and ear cup size of the HD650 and Eikon (as well as that of certain planars) seems to be more or less consistent with the law.