i use an ibasso D10 with my sony X, with an ALO line-out dock. the walkman's internal amp does provide a clean sound, but it doesn't have enough power to make my ultrasone pro 750's loud, even at max volume. i chose the d10 because it has a good DAC too that i use with my laptop. if i didn't need the DAC, i would probably have gone for a headstage arrow or pico slim.
a sidenote that may be of interest: when plugged into an amp from the line-out, the volume is disabled in the sony, but the sound processing effects and EQ still work. i'm assuming because the sony has a digital amp which modifies the sound before it gets transformed into an analog signal.
edit: the ibasso d10 also helps get rid of some hiss/noise that i notice when using my orto e-q7's straight from the headphone out.