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