Best RIPPING Format

I don't know if they have addressed this issue in the later versions of the firmware or not but ripping in WAV did not embed the metadata (id tags) in the WAV files. Instead the metadata was held in a separate companion .id3 file for each and every track.
I would suggest ripping one CD in WAV, copying it to the PC and then checking it with a tag editing program that will read WAV files, like mp3tag.
If there aren't any tags in the WAV files then this will present problems in the future when you want to copy or convert them to use on another device.

Personally I would rip in FLAC, it definitely will have embedded tags when ripped on the X50(D) and as it is lossless it can be converted to any other lossless format, plus it has wider support than most other lossless formats so will play on a wider selection of devices.
If however you are invested in the Apple ecosystem, then ALAC is probably the way to go.