Sounds like Lindisfarne has direct experience with this matching. I do not but have never heard a smallish stand mount or floor stander sound bad with the Nait 5. The XS has more steam than the 5, and so should be able to starch the ATC's shorts for most types of music and at most listening volumes.
Naim doesn't post many specifications on their website, so it is difficult to compare peak current capability or dampening factor for their amplifiers, but from personal experience, their power ratings are conservative compared with other makes. Generally, Naim amps seem to behave somewhere in between tube and other solid state rigs in terms of speaker driving capacity per advertised wattage, i.e. they seem to do more with less.
Naim doesn't post many specifications on their website, so it is difficult to compare peak current capability or dampening factor for their amplifiers, but from personal experience, their power ratings are conservative compared with other makes. Generally, Naim amps seem to behave somewhere in between tube and other solid state rigs in terms of speaker driving capacity per advertised wattage, i.e. they seem to do more with less.