Anybody tried Naim integrated with ATC speakers?

Might a smallish one, in wattage terms (ex: Naim, Sugden, LFD) sound better than a bigger one ( ex: Krell, Musical Fidelity) with the sensitive AND less sensitive speakers, or do the insensitive speakers always need more wattage, in order to have a good sound?

Small watts is easier to make then big watts - so for the same price a lower wattage amp will likely be better quality. However, a good amp will sound good whether it is 25 watts or 150 watts.

ATC's are a fairly easy load to drive (no nasty dips in impedance) but the 20 is not very efficient - so that is why you will need a beefy amp if you want realistic volume levels up to 105 db SPL.

I'd say you are at the minimum power that you would want. The problem with ATC is that they play very cleanly (low distortion) without effort and, as a result, there is always a tendency to increase the volume. A NAD M3 would go well with those speakers..