My take would be: Even the most basic 252MP based amp is likely going to be "very good" with a speaker like this. Peronally, if you don't need the geek points / bragging rights, I'd go with that and be happy. Perhaps put the money saved to work elsewhere in the system. (Dirac/EQ, Room treatment, DAC etc). Or go on a holiday. Either way
To get technical, I think you can expect SINAD (Signal to noise + distortion) to be up to 10dB better with a decent NC400 build versus a budget MP. But how much this matters in your setup I dunno.
I've got NC500, 252MP, and 502MPs but I only once did some testing relating to the differences, and sadly that was only on entry level Dynaudio Emit 20s.
I swapped from a Crown DSi2000 to a 252MP and was like "ok, wow, this is noticably better" then I swapped to the NC500s and it was more "hmm.. is that different?" Maybe a better speaker would be more revealing of the difference in amps; but I'm pretty convinced that's not always true.
I also don't have any NC400 amps for reference. So maybe an absolutely top dollar one of those might be a step forward, but I somehow doubt it would give me another "wow" moment against the NC500s or even the 252MPs I've already got.
Sorry this reply is probably less helpful than I first thought it might be