Atom or Nova - how much power to drive Dynaudio Evoke 50's?

I’d say not by far the Atom, and not even the Nova is going to be able to drive those speakers properly.

Note that its 4 Ohms mean that in the most dynamic passages will fall frequently even up to the 2 Ohms, and there Naim does not feel comfortable.

My experience with the ProAC K6, also 4 Ohms nominal but even a better sensitivity than the Dynaudio, has been that neither the SN2 nor the SN3, both 80W in 8 Ohms, have been able to drive them satisfactorily, and in the end I ended up with a Musical Fidelity Nu-Vista 600, which is another story both for synergy with the boxes and by technical specifications, starting with the 200W at 8 Ohms and 400W in 4 Ohms.

In short, the Atom not by far, and the Nova, well, will work reasonably as long as the room is not too big, do not ask too much volume and/or do not ask for super dynamic compositions (EDM, symphony, reggae…).