Maybe this will help.
The amp is producing hum that the NightHawk is reproducing. Think of NightHawk as a monitor of what comes before it in your system. It can only move, which equates to making sound, if there is a signal present at the output of the amplifier to make it move (make sound). In this case, the sound is hum. Something before the NightHawk is humming.
With hum, there are only two possibilities-
1) It goes up and down with the volume control and is therefore gone with the vc all the way down or
2) It is at a fixed level, irrespective of vc position and therefore is audible with the vc all the way down, equally loud as when the vc is in listening positions.
If it is the former, it should be equally audible on all 'phones because it removes the headphone sensitivity issue from the mix and is simply present at a rate that is proportional to the music signal level- music gets quiet, you hear it. The headphone doesn't matter.
If it is the latter, then a more sensitive 'phone (e.g. perhaps NightHawk in your experience) will make the hum more noticeable because you will have less music proportionally to cover it up. With a less sensitive headphone, you will turn the vc up to reach the same listening level as NightHawk, but the hum is fixed so it doesn't go up. One could imagine a situation in which a headphone could be enough less sensitive than NightHawk that the hum would be (all but) inaudible.
In any event, something in your chain needs addressing, but it isn't the NightHawk.