I think for the X3 and X5 you can add an external DAC (coaxial digital output). I currently use an iPod Classic with a digital dock as a kind of simple music server and thought I'd change to an X3 when my iPod gives up the ghost.
Unfortunately I've never listened to one so I can't comment on the quality of its own DAC. As I have DACs in my receiver and headphone amps I planned to use those.
But anyway, my point is that if you aren't satisfied with the performance of the X3's DAC you could always get a Rega DAC later.