Am I right in suggesting that the entry-level pre/power combo would need an upgraded source to make the most of it?
Ideally, if buying source and amp from scratch, yes.
But the OP already has a perfectly respectable CD player that is not going to 'disgrace' itself or the system (to paraphrase Dave earlier). I have heard a Nait XS with an Apollo and also with a £1500 Primare i30 and it sounded fine (actually excellent) in both instances.
Yes, the CD5i and CD5X (now CD5XS) sounded better with with a Nait XS but the Apollo is a perfectly viable option.
If I had not been offered a good deal on my Nait 5i/CD5i, I would have gone for the Apollo/Nait XS system, but there were no deals on that combi at the time, so I would have to have spent almost £2k instead of £1496.
The Rega Apollo (and Saturn according to True Blue) seem to be exceptional in having good 'synergy' with Naim amps whereas many other CDPs don't.
If I were the OP I would go for the Nait XS (and Flatcap 2XS* at a later date when funds allow) and eventually save up for a CD5XS to replace the Apollo last of all. (CD5i is a tad better than Apollo but probably not worth upgrading to now given what the OP would lose on the deal.)
* Note that the Flatcap 2XS has two outputs and will therefore serve both the Nait XS's pre-amp section, and the analogue output stage of the CD5XS (or the older CD5X)
Rega P2 • Ortofon 2M Blue • Rega Fono Mini • Naim Nait 5i • Naim CD5i • Naim NAT 05 FM tuner • Rega R3 loudspeakers • Naim NAC-A5 speaker cable • Beresford TC7520 • Panasonic DMR-EX78 DVD/HDD • Chord Chrysalis, Crimson & Naim interconnects