Headfonia's Lieven has some good stuff to say about the Beyerdynamic A20, including news that will make headphone and earphone lovers rejoice.

“The A20 is not really supposed to be used with custom IEMs but while I’m at it I’ll just add them to the list. The Cosmic Ears BA4 is dead silent when connected and there is no hiss and no channel imbalance. Be careful with the volume dial however cause it will get loud fast.”

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.

Lieven concludes:

“what’s not to like? Mid-Fi doesn’t get any better than this.”

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.