You don't need the power for the Harbeths. But, you need a minimum of 200 WPC to power the ATCs. I've tried to run the ATCs with less than 200 WPC, and it doesn't cut it.
Either way, the more power you add, the better bass articulation and control you'll have.
Again, I'm not sure whether I'll go with the ATCs or the Harbeths. Both are wonderful speakers. One is a bit more polite and refined than the other. The other seems to do better with dynamics and has richer mids.