@Gazza makes a good point, you could look for decent trade in against ex demo units as they cycle through stock. Dealers tend to keep demo stock 9-12 months then pass them on, always worth keeping in touch with whoever you have local in that respect, a good dealer will be flexible enough to do a deal.
In terms of comparisons between systems, it’s a good question and a very common one!
When I went through the XS NAIT’s then on to a SN2 I also had a SN1 for a short time between the NAIT XS and NAIT XS 2. The SN1 had the two rows of facia buttons and also had an internal DAC, ultimately I ended up swapping it for a NAIT XS 2 and built up from there. I enjoyed the XS 2 a lot and added a HiCap to it and changed CD player around same time as well as moving from mostly Chord Interconnects to a Naim Hi-Line.
The SN2 made quite a difference I felt mainly having more power overall especially bottom end and a wider soundstage especially with a HiCap on it driving the Preamp in the SN2.
It’s probably fairest to compare the NAC202/NAP200 to the SN2 as they are at closer performance levels, if budget space was/isn’t an issue then the 202/200 is a more musically rewarding system. It needs at least a HiCap (can use a SuperCap) and ideally you’d be looking at a better source, certainly for digital like an NDX 2 for example, the NDX 2 tends to end up with an XPS DR powering it, so you can see you could get to 5 quite expensive boxes even low down the ladder on the pre/power range.
From there you can move to a NAC282 easily enough and that in fact was my “old” plan that from a 202/200/HiCap I would replace the preamp with a 282 (powered by HiCap) then replace the HiCap with a SuperCap then replace the 282 with a 252 (powered by the SuperCap) then replace the NAP200 with a 300 which has it’s own PSU (300 PS) which gets you to 6 boxes and a temptation to go to more!
This is all based on a passive setup, if you then moved to an active system and used a SNAXO crossover you can then separately feed each speaker driver with it’s own NAP either 2 way or 3 way.
So you can see if your appetite and wallet allows you can take this a long way and that’s just on a path from the XS range and up through the Classic range!
Whatever you have there’s always something bigger and better above it unless you’re Louis Hamilton or Richard Branson, so the advice to get the source specced up is probably a valid one, it does mean you won’t have anything to change for maybe months to come but it’s probably worth doing things that way in the longer term.
In the meantime yes you can sell on your Flatcap and get a HiCap, that will improve the amp you have and probably the next couple after that! It will tighten up the bass and open up the sound a little on your NAIT XS 2, beyond that you can use it on a SN, a NAC202 and even a NAC282 either on it’s own or as a pair on the NAC282, it’s a very flexible PSU and for what it costs, even new it’s good value.
If you look out for a used one aim to get one around 2-6 years old to maximise price against value. Avoid older non DR ones, they look the same but lack the benefits of the DR (Discrete Regulator) technology. The suggested maintenance interval on amps and power supplies is 10 years so again buying used kit that’s less than 10 years old means you avoid a possible service bill, they can be expensive depending on the kit you are servicing, that said Naim are very good at servicing and repairs and apart from some products where spares aren’t available, like DVD players, they can and do servicing almost anything they’ve ever made.
I don’t know what Interconnects you use but a Hi-Line works well and is worth a try, your rack is a part of the system as well so you could also look at a used Fraim Lite for example.