I would get the 48v power supply regardless, as you might change the speakers or room in the years ahead.
Use this to calculate how much watts you need. Try to get to 96dB to be able to listen a bit louder. Consider that this calculates one speaker, you need to add +3dB if you use two, like most of us, so you aim for at 93dB, more is better:
It also depends on how much you put into the RCA connectors, but let's just assume it's a perfectly implemented streamer that outputs clean 2V max.
Yamaha states 87dB sensitivity for the NS-333. In my room i would sit 3m from the speakers and they would be 80cm from the wall. To get to 93dB (plus 3dB for the second speaker to reach 96dB), i would need 18W.
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This of course is just estimated, as it depends on the source output, the music itself and so many other factors. My guess: It's enough for loud music, but you won't bring the house down with this speaker/amp combination. It would be more than enough for my taste.