The only one of the three I have heard is the CDX, which I had with an XPS. My understanding is that the CDi is cut from the same cloth as the original CDS, which I've heard many times. There is no guarantee that if you liked the CDi you'll like the CDX. I thought the CDS was the best player I'd heard, one of the very few CD players I thought actually played music, but I couldn't get on with the CDX. Even with the XPS.
I also had a CD5XS, which was said to be closer to the top players than the budget ones it shared a case with. I really wanted to like that player but it was laughably behind the machine I was trying to beat.
The Rega Saturn-R. I'd like to hear this player against an original CDS because I think it would be very similar and might beat it. It's certainly the best CD player I've ever owned. Not because none of the others did anything better but because it gets nothing dramatically wrong. It sounds more like a turntable than any CD player I've heard and gets music out of absolutely any CD. Even the ones you thought were terrible productions. It makes you forget about the machine and just love the music. I prefer it to my RP10 turntable. In some ways the turntable is technically better but it's not as rounded and can't make every LP sound nice.
I'm not trying to hate on Naim CD players here. I like them, I've had three and I love the looks and ergonomics. The Saturn-R has flaky control software and the tight disk tray is a pain but I can't argue with the sound. It's just gorgeous, but not in the lush, soft way of some players. The fact that it has a full suite of digital inputs as well is icing on the cake!
Manufacturers have not been twiddling their thumbs in the intervening years and there is some pretty good stuff out there. A newer player would also remove the question of serviceability. Rega used a really cheap transport on the Saturn-R, the same one they use in the Isis and Apollo-R, and my understanding is that they have tons of them. All I'm saying is that the machines you're looking at are very old and if you widen your net a bit you might get something that sounds better and doesn't have any back-up worries.