People buy amps specifically for low impedance headphones because not everyone likes the way headphones sound with resistors. Even headphones that perform well with the resistors show a point of diminishing returns or too much of a good thing when adding more. An amp like the crack already has a bit of output impedance (about 60ohms If memory serves) BUT has difficulties driving a low impedance load.
Crack+HD600/650 was a realllly nice pair.
Im not sure which way Id go between the DT770/D2000. I think that the D2000 has a slight edge for sound, but it dosnt isolate very much at all.