NAIT 3 to XS3 of any use? No regrets, but don’t underestimate how good the 5i is, particularly if your speakers are an easy load. No regrets here, but source upgrades/additions I made later were more noticeable. E.g. adding a Hugo mk 1, for £500, even driven by a mobile phone initially was fairly amazing.
What source/s do you run?
I did have the opportunity to try a 5i as well as my NAIT 3 either side of a service on my 3, and the 5i sounded very good, a definite step up. So I’d thoroughly discount a source upgrade before going for the XS.
Sources are generally fed through a Rega DAC or a California Audio Lab Sigma hybrid tube DAC. All optical from an Airport Express, itself an excellent DAC if using the 3.5mm RCA output.
There’s also line output via a 30-pin AV cord that connects the iPad stuffed with lossless CD files… that is also unplugged from the AC… any chance I have to use a DC powered source I take it.