That depends on how far you are sitting away from them and how loud you want peaks, like an orchestra crescendo, to be. Do you have any idea what you want these two parameters to be. As a reference, I think THX standard is somewhere around 102dB for peaks.
One thing you want to consider with the amp is during what percentage of it's power rating in it linear/low noise. For tube amps that seems to be 50% of it's rated power. SS amps seem to be in the 10-20% range. This is what I have seen in noise graphs for tube amps and have been told regarding power amps.
It's not that you will use all that power. It is to make sure not only does the amp never clip, but also that it always just cruises along.
All that said, as Jay says, you can easily run them with a good 50wpc amp if you are not a long way away from them and/or do not want to play them crazy loud. It's just that more amps are better for safety and clarity.