In the US this is well accepted, in Spain it is possible but it is not often done, in any case I would like to receive some opinions from you, the experts
First of all, I think you are being very modest when you say you have a "very modest" system! Those Focal Arias are very fine speakers--I run a pair of them in my home theater; I've listened to music on them and they are very nice.
Secondly, you haven't really said what your objective is, but I take it that you are looking for "better sound," or at least a different sound. To that end, upgrading the DAC is probably one of the LAST things I'd do, though the PEQ capability on the ADI-2 could at least enable you to play with the sound a bit.
I'm not familiar with your amp, but in general Class D amps have a very neutral, accurate sound. People who like the warm, fuzzy euphonic distortion produced by tube amps often describe Class D as "sterile, cold, thin, ...". So, if you are looking for something different, moving to a "warmer" amp might be a way to go. Whether "different" is really "better" is a matter of subjective opinion!
Before buying any new equipment, however, I'd play around with speaker placement, and perhaps apply some room treatments to the corners, first-reflection points, and behind the main listening position (if applicable) to reduce any unhelpful resonance in your room.