I haven't heard the CD9, but I do own a K-01X. At this point I wouldn't want to be without a USB input that can't handle DSD128 native or upsampled and custom filters from Roon's DSP engine. The K-01X does that, and also synchronizes to a 10mHz master clock to further improve performance. If you're considering migration from discs to files and streaming, the K-01X offers the best of both worlds.
Esoteric K-01 vs ARC CD 9
Supposing both players had the same pre-amp, say an ARC Ref 6, which one would you prefer? The K-01 can also be played directly to amps--how would it do alone against the ARC combination, Redbook CD only?