IF all is the same and only difference is the speakers used, dispersion pattern differences between the old and current speakers would most likely account for differences.
First though I would switch left and right channels to see if that reverses the soundstage. If so could be something different in one channel versus the other somewhere upstream and not the speakers or room.
Wider dispersion patterns tend to work well in more chaotic rooms with strange acoustics. THings will tend to even out better that way. Maybe the old KEFs had better dispersion than newer ATCs? Just wondering.
If it turns out to be the speakers and not the electronics, might consider a change at that point to find something better suited to the room.
First though I would switch left and right channels to see if that reverses the soundstage. If so could be something different in one channel versus the other somewhere upstream and not the speakers or room.
Wider dispersion patterns tend to work well in more chaotic rooms with strange acoustics. THings will tend to even out better that way. Maybe the old KEFs had better dispersion than newer ATCs? Just wondering.
If it turns out to be the speakers and not the electronics, might consider a change at that point to find something better suited to the room.