Speaker wants 100 watt RMS. Is 40 watt tube OK?

90 dB sensitivity, 8 ohms, you should have no problem as long as you don't have a huge room.

You’ll be fine. 

otherones, depends on what you like listening to and how loud. If you are a stadium  rocker forget it. Jazz, classical and light rock at moderate volumes no problem. Personally, I think you are making a mistake. Save a bit longer and get a more powerful tube amp if you have to have a tube amp. 

Ripple

90db efficiency only tells one part of the story.  How the speaker’s impedance changes over the entire frequency range is very important for tube amp matching. 

I have PrimaLuna mono-blocks that put out 70 watts per side, and 92 DB speakers in a moderate room, and I'm quite satisfied.

They can play much louder than I want them to. I like jazz, as opposed to rock, if that makes a difference.

No matter what anyone says, 40 watt tube approximates 100 watt SS. The size of the room is the dominant factor in this equation.