Great, if you could demo anything from each brand that would be best, as basically all Proac and ATC 2-ways have the same house sound.
ATC recommends 75w min for the SCM19/20; it has 85/886db sensitivity vs the D2’s 88db, so the Proac will be louder at the same volume level. They both have similar impedances and won’t dip too low; an easy load in terms of current demands for any solid state amp. On the flip side, the ATCs will have higher power handling; less distortion and clipping when you want to crank it.
Another big difference is bass. Being ported, the Proac will reach lower beore its tuning frequency but then bass will fall steeply below that. Being sealed, the ATC will have better transient response (tightness), not going as low, but its rolloff is much shallower. I’d say the Proac should give you more bass down to 40hz before that sharp rolloff, the ATC about 50hz. The ATC will be less upset by acoustic issues from proximity to room boundaries, and the waveguided tweeter should have better off-axis response. It’s generally easier to integrate a sealed speaker with a sub if you go down that route in the future.