Nighthawks sound great on Audeze Deckard, a Project Ember, Aune T1 mk2, Asgard 2, Cayin C5, Fiio E6, Audioquest Dragonfly, Schiit Magni 2 Uber, O2, hell, even my iPhone 6 plus and Windows 640XL, anything.
It doesn't really need an amp as it has a ridiculously low impedance of 25 ohms. It's a very smooth headphone and doesn't really need a tube amp to smooth things out. If you want Tube amp+DAC for it, I'd suggest the Aune T1 mk2. If solid state, get Modi/Magni. Asgard 2 sounds nearly identical to the Magni 2 Uber, but it has more power for high impedance headphones, which the Nighthawk is definitely not, so you won't need it. Magna 2 is more powerful on low impedance headphones.