Vince said: Although other portable USB DAC-headphone amps work well with Apple devices, the ADL by Furutech X1 is a unique device that adds 24/192 capability via USB. It sounded splendid at all resolutions, and with every device that I connected it to. At $479, the X1 is pricey -- but you get a versatile, classy-looking device with exceptional sound quality to match its good looks.
Read the SoundStage! Xperience review.
The gist: Competition for Astell&Kern.