The XD models have the new proprietary Esoteric discrete DAC on board, while the prior models use AKM D/A chips. They sound very different to my ears.
Personally I prefer the new discrete DAC. I always felt like the AKM-based models were beautiful in their way but not really to my liking in terms of balance. A little too lit up and almost cold in a way. You could build a system around them and make it sound amazing but for my gear it wasn't a great match.
That doesn't mean you will feel the same way as me though. Maybe for your ears and system the AKM-based players and DACs are superior. Really need to hear them for yourself to decide because again, the difference is pretty significant.