If it helps at all, the first time I ever heard Naim’s iconic SBL speakers was with a CD5i, Nait XS and NACA5 speaker wires. The electronics were forerunners of your gear but you get the idea.
The “natural” CD partner for an XS amp would be the CD5XS. My entry into Naim-world was an original Nait XS and CD5XS. The latter is long discontinued and likely unserviceable if the mech goes, so I’d be wary of jumping that way even second hand.
As my system evolved I added a nDAC to the CD player … a great upgrade. That was using the CD5XS as transport, so you might want to consider the nDAC with a non-Naim transport. Some forum members have gone that way and a bit of searching should provide a list of possible transports.
Everybody’s ears and tastes are different - that said, I’ve got my UQ2 and Neat Iotas cranked up to eleven playing Steely Dan’s Show Biz Kids. Good sounding music that gets up and moves is so enjoyable.
I use a CD5Si with my XS3. Works very well IMHO. Compared with my Arcam D33 DAC I would say the highs a less extended and the soundstage is smaller. However, on certain music, such as rock, the CD5si has the ‘boogey’ factor and it can also take the most aggressive edge off some music. My only negative is that it does not have a digital out to feed an external DAC (such as my D33) if I wished for a different presentation.
Given the lack of runway for Naim CD players (I have a CDX2.2 which I love) it might be a better strategy to get something out of brand, I have a Denon DCD1700NE which I have into the Nait 50 and they go very well together. It’s an absolutely fabulous sounding player that has all the drive and timing of Naim imo.
It’s worth trying a CD5si if you can (ideally in your system) to see if you would be happy with it - it would be a largely aesthetic and functional match - I say largely because the front panel of the 5si series has a different finish.
However, I would fully understand going a different route. If, for example, you plan to purchase an ND5 XS2 streamer, then a good quality CD player/transport feeding the digital inputs might be a preferable option. Alternatively, as Adam mentioned above, a there are alternative quality CD players in that price bracket, some of which also serve as external DAC for more budget orientated streamers.
Personally, unless bought cheaply enough, I would be weary of Naim players that are no longer supported - a real shame, but it’s a risk unless the player was cheap enough for you to give up on it if the mech were to fail.
I am too: CD5si into Nait XS3 plus ND5 XS2 streamer (for radio and Tidal) and NAC A5 speaker cable into Neat Mystique Classic speakers. The Naim boxes were bought new together in 2019 and I’ve had no reason to consider upgrading them since.
I can still purchase a brand new CD5Si that would come with the Naim warranty and that’s quite appealing to me as cd players are a huge risk at the moment