I don't agree with the statement: 'an amp is definitely necessary'.
The ASG-2 isn't an easy earphone to drive, but it isn't hard, either. Modern hard ware, especially the likes of iPod nano 7 or so, should render all the resolution this earphone needs- unless you are keen on more distortion. In which case, I have no argument. Plus, there are very few amps that meet the necessary requirements of earphones such as these.Those amps seem to have seen their heyday back in 2007 or so. Recently, amps are aimed at higher Ω earphones/headphones.
voltage/volume: no doubt you will get more with an amp. Absolute resolution/sound quality (actual, not figurative) is NOT a given with an amp.