Sandisk Sansa Clip Zip 4GB, with a 32Gn micro-SD memory card.
Install the Rockbox firmware.
The Clip Zip can decently drive my 62-Ohm Sony MDR-V6s, so should be ok with the 50-Ohm ATH-M50X.
You could always daisy chain a FiiO E11K headphone amplifier ($65) to the Clip Zip optional).
FiiO Will be coming out with the X1 in the near future, $100.