If you love it and are happy with how it sounds then I’d suggest you don’t worry about it. You’ve been perfectly happy for two years, which says it all really. I certainly wouldn’t add a 100. You are probably best saving up for something like an NDX2/Supernait 3 if you want to upgrade.
I do love it, but I thought a “small” investment with the NAP100 could increase my pleasure I have an opportunity to get a second hand NAP100 for 700€.
The NAP 100 will add little, the NAP 100 is 50 watts , the Atom 40 watts.
The NAP 100 was designed more to go with the Atom’s predecessor of the UnitiQute or the DAC V 1 preamp, which would make it a super little combo for reasonably efficient speakers.
It is a common question on this forum on how to take the Atom forward, I think many who heard them together say the Atom/NAP200 is a good match (not heard them together myself) .
Don’t! I bought my pre-loved NAP200 for 750€ and that works like a charm with the Atom and my Dynaudio loudspeakers. I am very happy with the result and it makes great music. More ‘confident’ than with the Atom alone if I would need to describe the difference.