Headfonia's Lieven has some good stuff to say about the Beyerdynamic A20, including news that will make headphone and earphone lovers rejoice.
It seems that more and more home headphone amp makers are designing circuits that accommodate earphone listeners. Antelope Audio's Zodiac is another that, at least from a volume balance/noise perspective, very well with sensitive earphones.
The one gotcha is that the A20's output impedance is high, which may render the output of certain low Ω headphones (even some Beyerdynamic) subject to aberrations in frequency response, stereo performance, among others. At least the noise floor is low.