OK & understood - but, in the world of hi-fi (and it is/was a trait of some B&W speakers), smaller size doesn’t mean easier to drive - strangely larger box speakers, although having more drive units, can be more efficient and easier to drive.
On the face of things, and I don’t think you are the first to encounter this (and there’s no guidance in the Quick Start Manual/Guide from what I can see), I suspect the combo of the speaker cable (not just type of cable but also length) & speakers isn’t an easy load for the Atom, especially where the volume is pushed a tad. Yes, it may have been OK for a long time but things do change.
I wish you well in resolving this.
FYI, have a look here to understand what Naim’s broader requirements & approach are re speaker cables/length etc,.