No comparison of the two by me. The 840C is my primary player. I find this player provides much detail within the music. I listen to alt rock, mostly. The 840C's detail has alotted me interesting insight into my CD collection. I better hear/recognize signature musicality of bands, from recordings spanning their careers. For example I played a CD of Rolling Stones "Exile on Main St" which I hadn't played in many years. As I listened just the other night I discovered/heard a style of guitar playing that quickly reminded me of later releases from the Stones like "Tatoo You". This is subtle stuff (not Mick's vocals) that I hadn't really heard in their music before. I used to think early Stones was quite stylisticly different than later, but now I'm more aware of a tie between the old and its offspring. The 840C allows me to hear these subtleties. It gives me more reason to listen and enjoy.
As mentioned, I have never listened to a Cary cd player so I apologize for responding given I haven't compared the 2 players. One thing I have noticed, both of these players and other CD models from Cary and Cambridge come up for sale quite often on the 'Gon, though I wouldn't consider these manufactures "volume" producers like a Sony, Marantz, Rotel or NAD (I could be wrong). Maybe there is a lot of auditioning-by-purchase of Cambridge and Cary players going on. Of course the 840C has the Robert Harley accolades. The Cary CDP-1 received recognition in Stereophile.