I have a couple of pairs of Harbeth's, and they loves watts, the more the better!
Anything under 150w will not release the full potential of the speakers, just look at some Harbeth shows and what Alan uses to demo his speakers, high wattage Hegel's!
Watch this from 6:00 min:-
Forget the Naim SN3, they just don't work well, I sold mine when I got my Harbeths...
I went to a Harbeth demo with AS in charge.
Amps were some 200-250w American mono blocks.
Performance was a disaster,
Bloated bas
Shut-in mids
Really un-engaging boxy
I think even AS noticed judging on panic in his eye, nobody to brainwash as people left room fast
Reading watt specs and your out in the desert, not many amps control speakers as well done as a Supernait.