My guess is that 250 will drive the 40 fine. The Naim sound is predominantly from the preamp so if you decide the actives then Naim sound will be preserved by the 272. Imho the actives will provide better control and sound quality from the speakers. Saying that the passives are also excellent too so you can't go wrong either way.
When comparing speakers make sure you demo in same room with same positioning as room acoustics, amp, and cables can make a big difference. You want to make sure everything is right. Underwhelming, sterile, and cold would not be words I would use to describe the ATC speakers I have listened to. They are very honest to the rest of the system and music and will reveal any shortcomings. If the music is warm and cosy then that is what you hear.
Read Jason Kennedy of hifi+ mag review of Scm40a with 272 online, summarised as the best two\three box system he has heard.