Some thoughts and questions:
1- I assume you are using the Dynaudio 40s (quite small but punchy and a good quality speaker) – what is it that’s promoting the idea to change these e.g. do you feel they are limiting in some way? Size of room, the sound level you like to play at?
2- Have you the space (and Fraim) to install a 500DR, given it’s 2 units and it’s vital not to have the 2 x burndy cables hanging down on to the floor and/or touching other cables, as this can generate sub-optimal bass – which could do your room no favours?
3- you say final 500-series item (and noting you are in Singapore?) and the use the term ‘upgrade next’, suggesting you may have more plans to upgrade down the line?
The reason this is important is:
a- You don’t want to be over-speakered (in hi-fi language) and running 808s/Scalas (assume the 3-ways) with a 250DR isn’t sensible. @nicnaim is spot on here. IMHO, a 250DR simply won’t do these speakers justice.
b- you could up-speaker now, but you will find using a much better amp should change things much for the better – and after this you can select a speaker which marries in your room to this (the room/amp/speaker trinity).
Sometimes less speaker is better for a room and for matching to an amp e.g. you could be better suited by the lesser Focals, another pair of better Dynaudios?
4- you don’t mention whether this would be new or pre-loved kit? Obviously, there is some debate about the qualities of the NC 350 amps against the existing, and far more expensive(!) 500DR.
Are you fixed on the 500DR, as the 350s could free some budget here?
Bottom line in all this, it’s generally the rule that a better amp in to modest speakers sounds far better than a modest amp in to better speakers, primarily as the latter will play only what they are fed and a modest amp will not control the speakers optimally.