I auditioned the special 40 and the Evokes and picked the Evokes (so that you know the differrence). My amp is comparable.
So far my sub experience has been a mixed bag. I tried the SVS and Klipsch entry(ish) level models, placed them everywhere and found that with some music it makes a net positive difference. Otherwise it’s a sonic mess. So I am not sure your solution is a sub, The Special 40s are even more musical and complex than the Evokes and it is not easy to integrate them without an amp with built-in crossover control. If the NAIM does that, that’s a big plus.
I found that the greatest improvement for me is the source, my turntable, the vinyl, and I am considering the DAC. I will try higher-end subs (KEF, REL) but unless I listen to perfect and "lucky" arrangements where the drum finds its own space in the soundstage, I am very pessimistic about adding a sub.